ASoC: tegra_wm9712: assume CONFIG_OF

Tegra only supports, and always enables, device tree. Remove all runtime
checks for DT support from the driver.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>