== mc->rshift

is stereo, within single register:
mc->reg == mc->rreg && mc->shift != mc->rshift

is stereo, in two registers:
mc->reg != mc->rreg

The patch provide a small inline function to query, if the mixer
is stereo, or mono.

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