ASoC: core: Change SOC_SINGLE/DOUBLE_VALUE representation

SOC_SINGLE/DOUBLE_VALUE is used for mixer controls, where the
bits are within one register.

Assign .rreg to be the same as .reg for these types.

With this change we can tell if the mixer in question:
is mono:
mc->reg == mc->rreg && mc->shift == 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>