mfd: abx500-core: Fix sparse warning

Fix sparse warning:

drivers/mfd/abx500-core.c:159:38: warning: Using plain integer as NULL pointer

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>