devm_gpiod_get currently has an optional parameter to set initial
direction and value for the requested gpio. Make use of this to simplify
the driver and make it not fail to build when this parameter is made
mandatory (which is scheduled for 4.3-rc1).

Moreover use the _optional variant of devm_gpiod_get to simplify error
handling (which also gets more strict for free).
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>