mmc: card.h: Use NULL instead of 0 for END_FIXUP

Fix the following sparse warnings:

drivers/mmc/card/block.c:2421:9: warning: Using plain integer as NULL pointer

drivers/mmc/core/quirks.c:69:9: warning: Using plain integer as NULL pointer

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>