mfd: arizona: Correct handling of device tree gpio defaults

When setting GPIO defaults we are required to make a distinction
between writing 0x0000 to the registers and leaving them untouched.

When we receive between 0x0000 and 0xFFFF (inclusive) from either
Platform Data or Device Tree, we should write the provided
configuration to the device. Conversely, when we receive >0xFFFF we
should leave the device configuration at its default setting.

This patch fixes a bug and ensures that configuration 0x0000 isn't
mistakenly written when the intention was to keep the default one.

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