sound: tegra_wm8903: Adjust to of_get_named_gpio() change

of_get_named_gpio() was changed to return -EPROBE_DEFER in case of
gpios not probed yet. This patch adjusts tegra_wm8903 to this.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>