regulator: cleanup regulator_ena_gpio_free()

Since only regulator_ena_gpio_request() allocates rdev->ena_pin, and it
guarantees that same gpiod gets same pin structure, it is enough to
compare just the pointers. Also we know there can be only one matching
entry on the list. Rework the code take advantage of the facts.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/3ff002c7aa3bd774491af4291a9df23541fcf892.1597195321.git.mirq-linux@rere.qmqm.pl


Signed-off-by: Mark Brown <broonie@kernel.org>