mfd: max77686: Use NULL instead of 0

'data' is a pointer and hence use NULL instead of 0.
Silences the following warning:
drivers/mfd/max77686.c:49:50: warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>