mfd: arizona: Correct type of gpio_defaults

gpio_defaults needs to be specified as an unsigned int rather than an
int, because the intention of the DT binding is that all out of range
values for a 16-bit register will cause the defaults to be used,
however, if gpio_defaults is an int then values that are larger than
INT_MAX will become negative numbers and be written out directly to the
hardware. As no where in the code replies on gpio_defaults being an int,
the simplest fix is to just change it to unsigned.

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