ASoC: tegra: call pm_runtime APIs around register accesses

Call pm_runtime_get_sync() before all register accesses; the HW requires
clocks to be running when accessing registers.

This hasn't been needed to date, since all register IO was performed
while playback was active, and hence the ASoC core had already called
pm_runtime_get(). However, an imminent future commit will allocate and
set up the FIFOs and routing during probe(), when that "protection"
won't be in place.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Brown <broonie@linaro.org>