pmdomain: core: Convert genpd_power_off() to void

At some point it made sense to have genpd_power_off() to return an error
code. That hasn't been the case for quite some time, so let's convert it
into a static void function and simplify some of the corresponding code.

Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250417142513.312939-2-ulf.hansson@linaro.org