ASoC: improve usage of gpiod API

Since 39b2bbe3 (gpio: add flags argument to gpiod_get*() functions)
which appeared in v3.17-rc1, the gpiod_get* functions take an additional
parameter that allows to specify direction and initial value for
output. Simplify drivers accordingly.

Also there is an *_optional variant that serves well here. The sematics
is slightly changed here by using it as error checking is more strict
now: If GPIOLIB is not enabled an error is returned instead of just
ignoring the gpio. On one hand this is bad for devices that don't "have"
the respective gpio as the driver is failing now. On the other hand
there is no means to assert that this gpio is really not needed or if
only the driver to control it is not available. The latter is a real
reason to fail and so it's defensive to fail here, too.

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