mfd: palmas: Assign the right powerhold mask for tps65917

The powerhold mask for TPS65917 is different when comapred to
the other palmas versions. Hence assign the right mask that enables
power off of tps65917 pmic correctly.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>