mmc_spi: mmc_spi.h should include linux/interrupts.h

Since mmc_spi.h uses irqreturn_t type, it should include appropriate
header, otherwise build will break if users didn't include it (some of
them do not use interrupts).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>