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

commit 4f86a724 ("power: regulator: denied disable on always-on
regulator") throws an error when requested to disable an always-on
regulator. It is right that an always-on regulator should not be
attempted to be disabled. But at the same time regulator framework
should not return an error when such request is received. Instead
it should just return success without attempting to disable the
specified regulator. This is because the requesting driver will
not have the idea if the regulator is always-on or not. The
requesting driver will always try to enable/disable regulator as
per the required flow. So it is upto regulator framework to not
break such scenarios.

Fixes: 4f86a724 ("power: regulator: denied disable on always-on regulator")
Reported-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>