ASoC: sta32x: use devm_gpiod_get_optional for optional reset gpio

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.

Also there is a variant to find optional gpios that returns NULL if
there is no gpio instead of -ENOENT.

Make use of both features to simplify the driver.

This changes behaviour if gpiod_get returns -ENOSYS which is the case if
CONFIG_GPIOLIB is not enabled. This is a good change because without
GPIOLIB there is no way to determine if the reset gpio is specified in
the device tree. And if it is it must be handled, so erroring out is the
right thing to do.

Furthermore this is one caller less that stops us making the flags
argument to gpiod_get*() mandatory.

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