ASoC: tegra trimslice: assume CONFIG_OF, and other cleanup

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

Also, some minor changes so that the probe() body more closely resembles
other drivers, for easier comparison.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>