ASoC: omap-mcbsp: Single macro for st channel volume set/get

Since we always need to have set and get callbacks for McBSP sidetone it
makes sense to combine the two macro to create the two callbacks.

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