mmc: Fix crash in mmc_block on 64-bit

Fairly simple. "dev_use" was being allocated as a zero length array
because of bad math on 64-bit systems, causing a crash in
find_first_zero_bit(). One-liner follows:
Signed-off-by: Ben Collins <ben.collins@canonical.com>
Acked-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/mmc/card/block.c

@@ -46,7 +46,7 @@
 #define MMC_SHIFT	3
 #define MMC_NUM_MINORS	(256 >> MMC_SHIFT)
 
-static unsigned long dev_use[MMC_NUM_MINORS/(8*sizeof(unsigned long))];
+static DECLARE_BITMAP(dev_use, MMC_NUM_MINORS);
 
 /*
  * There is one mmc_blk_data per slot.