regulator: isl6271a: Use NULL instead of 0

init_data is a pointer. Use NULL instead of 0.

Silences the following sparse warning:
drivers/regulator/isl6271a-regulator.c:133:44:
warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>