regulator: Check status before disabling

When disabling regulator when they are unused, check before is they are
enabled.
While here don't check the enable_cnt on the regulator entry as it is
checked by regnode_stop.
This solve the panic on any board using a fixed regulator that is driven
by a gpio when the regulator is unused.

Tested On: OrangePi One
Pointy Hat to: myself
Reported by: kevans, Milan Obuch (freebsd-arm@dino.sk)