mmc: core: resolve divded by zero panic

With one special SD card, below divide by zero error observed:
...
[    2.144300] divide error: 0000 [#1] PREEMPT SMP
[    2.148860] Modules linked in:
[    2.151898]
[    2.152685] Set up 4031 stolen pages starting at 0x0001f000, GTT offset 0K
[    2.157330] Set up 0 CI stolen pages starting at 0x00000000, GTT offset 131072K
[    2.167581] Pid: 5, comm: kworker/u:0 Not tainted 3.0.8-138216-g974a2aba #1
[    2.169506] [drm] PSB GTT mem manager ready, tt_start 4031, tt_size 28737 pages
[    2.169906] [drm] SGX core id = 0x00000000
[    2.169920] [drm] SGX core rev major = 0x00, minor = 0x00
[    2.169934] [drm] SGX core rev maintenance = 0x00, designer = 0x00
[    2.197370]  Intel Corporation Medfield/iCDKB
[    2.201716] EIP: 0060:[<c1697ca6>] EFLAGS: 00010246 CPU: 1
[    2.207198] EIP is at mmc_init_erase+0x76/0x150
[    2.211704] EAX: 00002000 EBX: dcd1b400 ECX: 00002000 EDX: 00000000
[    2.217957] ESI: 00000000 EDI: dcd5c800 EBP: dd867e84 ESP: dd867e7c
[    2.224214]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
[    2.229605] Process kworker/u:0 (pid: 5, ti=dd866000 task=dd868000 task.ti=dd866000)
[    2.237325] Stack:
[    2.239322]  dcd1b400 00000000 dd867eb0 c16a06da c1ab7c44 dd995aa8 00000003 00000000
[    2.247054]  00000000 00000000 dcd5c800 00000000 dcd1b400 dd867ef8 c16a1012 c1698b00
[    2.254785]  00000029 00000001 c194eb80 dcd5c9ec dd867e00 c1239b00 00000000 00000000
[    2.262519] Call Trace:
[    2.264975]  [<c16a06da>] mmc_sd_setup_card+0x1da/0x4f0
[    2.270183]  [<c16a1012>] mmc_sd_init_card+0x192/0xc40
[    2.275304]  [<c1698b00>] ? __mmc_claim_host+0x160/0x160
[    2.280610]  [<c1239b00>] ? __schedule_bug+0x50/0x80
[    2.285556]  [<c16a1b89>] mmc_attach_sd+0xc9/0x230
[    2.290333]  [<c169b6ef>] mmc_rescan+0x25f/0x2c0
[    2.294943]  [<c1274223>] process_one_work+0x103/0x400
[    2.300065]  [<c12670fd>] ? mod_timer+0x1ad/0x3c0
[    2.304756]  [<c169b490>] ? mmc_suspend_host+0x1a0/0x1a0
[    2.310056]  [<c127502d>] worker_thread+0x12d/0x4a0
[    2.314921]  [<c18fcfbd>] ? preempt_schedule+0x2d/0x50
[    2.320047]  [<c1274f00[    2.323976] ---[ end trace 5398ec2720494438 ]---
...

So, seems this bad SD card does not set valid value in related SSR / CSD register fields.
And then the driver will set card->erase_size to 0.
Then it triggered this divided by zero error when calculate card->pref_erase.

Submit this patch to fix the issue.
Signed-off-by: Yunpeng Gao <yunpeng.gao@intel.com>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/core.c

@@ -1753,7 +1753,7 @@ void mmc_init_erase(struct mmc_card *card)
 		card->erase_shift = ffs(card->ssr.au) - 1;
 	} else if (card->ext_csd.hc_erase_size) {
 		card->pref_erase = card->ext_csd.hc_erase_size;
-	} else {
+	} else if (card->erase_size) {
 		sz = (card->csd.capacity << (card->csd.read_blkbits - 9)) >> 11;
 		if (sz < 128)
 			card->pref_erase = 512 * 1024 / 512;
@@ -1770,7 +1770,8 @@ void mmc_init_erase(struct mmc_card *card)
 			if (sz)
 				card->pref_erase += card->erase_size - sz;
 		}
-	}
+	} else
+		card->pref_erase = 0;
 }
 
 static unsigned int mmc_mmc_erase_timeout(struct mmc_card *card,