ASoC: cs42l73: Fix mask bits for SOC_VALUE_ENUM_SINGLE

The mask bits values were wrong for the SOC_VALUE_ENUM_SINGLE for the mono mix controls.

Change-Id: I1ec252c8a34d08f3e2b24b6ebb55590ec93be145
Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org