[MMC] mmc_blk_probe() warning fix

drivers/mmc/mmc_block.c: In function `mmc_blk_probe':
drivers/mmc/mmc_block.c:467: warning: long unsigned int format, different type arg (arg 5)

We don't know what size sector_t is - cast it.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>