MIPS: BCM47XX: use fallback sprom var for board_{rev,type}

An SoC normally do not define path variables for board_rev and
board_type and the Broadcom SDK also uses the nvram values without a
prefix in such cases. Do the same to fill these sprom attributes from
nvram and do not leave them empty, because brcmsmac do not like this.
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4679/Signed-off-by: John Crispin <blogic@openwrt.org>

arch/mips/bcm47xx/sprom.c

@@ -652,12 +652,10 @@ static void bcm47xx_fill_sprom_ethernet(struct ssb_sprom *sprom,
 static void bcm47xx_fill_board_data(struct ssb_sprom *sprom, const char *prefix,
 				    bool fallback)
 {
-	nvram_read_u16(prefix, NULL, "boardrev", &sprom->board_rev, 0,
-		       fallback);
+	nvram_read_u16(prefix, NULL, "boardrev", &sprom->board_rev, 0, true);
 	nvram_read_u16(prefix, NULL, "boardnum", &sprom->board_num, 0,
 		       fallback);
-	nvram_read_u16(prefix, NULL, "boardtype", &sprom->board_type, 0,
-		       fallback);
+	nvram_read_u16(prefix, NULL, "boardtype", &sprom->board_type, 0, true);
 	nvram_read_u32_2(prefix, "boardflags", &sprom->boardflags_lo,
 			 &sprom->boardflags_hi, fallback);
 	nvram_read_u32_2(prefix, "boardflags2", &sprom->boardflags2_lo,