mfd: arizona: Use num_core_supplies in arizona_dev_exit

Currently we call regulator_bulk_disable with
ARRAY_SIZE(arizona->core_supplies), however this array may be larger
than the number of supplies actually used by the chip we are dealing
with. Use the provided num_core_supplies member instead, so that we only
disable supplies which actually exist.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>