ASoC: davinci-mcasp: Consolidate pm_runtime_get/put() use in the driver

The use of pm_runtime in trigger() callback is not correct and it will lead
to unbalanced power.usage_count.
The only place which might need to call pm_runtime is the set_fmt callback.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>