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>