mmc: sdhci-esdhc-imx: Use NULL instead of zero

Fix the following sparse warning:

drivers/mmc/host/sdhci-esdhc-imx.c:617:35: warning: Using plain integer as NULL pointer

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>