mfd: menelaus: Use NULL instead of 0

'mmc_callback_data' and 'vtg' are pointers.

Fix the following sparse warning:

drivers/mfd/menelaus.c:445:43: warning: Using plain integer as NULL pointer
drivers/mfd/menelaus.c:469:20: warning: Using plain integer as NULL pointer

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>