pmdomain: core: Simplify return statement in genpd_power_off()

Rather than using two if-clauses immediately after each to check for
similar reasons to prevent the power-off, let's combine them into one
if-clause to simplify the 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/20250417142513.312939-3-ulf.hansson@linaro.org