regulator: rk808: make better use of the gpiod API

The gpiod functions include variants for managed gpiod resources. Use it
to simplify the remove function.

As the driver handles a device node without a specification of dvs gpios
just fine, additionally use the variant of gpiod_get exactly for this
use case. This makes error checking more strict.

As a third benefit this patch makes the driver use the flags parameter
of gpiod_get* which will not be optional any more after 4.2 and so
prevents a build failure when the respective gpiod commit is merged.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>