mwifiex: fix compile warning of unused variable

We got a compile warning shows below:

drivers/net/wireless/marvell/mwifiex/sdio.c: In function
'mwifiex_sdio_remove':
drivers/net/wireless/marvell/mwifiex/sdio.c:377:6: warning: variable
'ret' set but not used [-Wunused-but-set-variable]

Per the code, it didn't check if mwifiex_sdio_read_fw_status
finish successfully. We should at least check the return of
mwifiex_sdio_read_fw_status, otherwise the following check of
firmware_stat and adapter->mfg_mode is pointless as the device
is probably dead.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/mwifiex/sdio.c

@@ -390,7 +390,8 @@ mwifiex_sdio_remove(struct sdio_func *func)
 	mwifiex_dbg(adapter, INFO, "info: SDIO func num=%d\n", func->num);
 
 	ret = mwifiex_sdio_read_fw_status(adapter, &firmware_stat);
-	if (firmware_stat == FIRMWARE_READY_SDIO && !adapter->mfg_mode) {
+	if (!ret && firmware_stat == FIRMWARE_READY_SDIO &&
+	    !adapter->mfg_mode) {
 		mwifiex_deauthenticate_all(adapter);
 
 		priv = mwifiex_get_priv(adapter, MWIFIEX_BSS_ROLE_ANY);