ASoC: Fix SOC_DOUBLE_R_SX_TLV volume mixer arguments

Remove unnecessary bit shifts.
Correct min value to match datasheet.
Num steps = number of steps between min and max.

Reported-by: Ryan Harvey <ryan.harvey@cirrus.com>
Signed-off-by: Ryan Harvey <ryan.harvey@cirrus.com>
Signed-off-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@linaro.org>