mfd: palmas: Reset pm_power_off if it is set for the device

If Palams supports the system power controller and pm_power_off
is implemented through the Palmas driver then reset the pm_power_off
in driver remove.

This will avoid the call of Palmas driver after removal of driver.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>