Revert "power: regulator: Return success on attempt to disable an always-on regulator"

This reverts commit e17e0ceb.

It is advised to return an error when trying to disable an always-on
regulator and let the consumer driver handle the error if needed.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>