Fix sparse warning:

drivers/mfd/abx500-core.c:159:38: warning: Using plain integer as NULL pointer
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NFabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>