ASoC: cs42l51: Fix enum ctl accesses in a wrong type

"PCM channel mixer" ctl in cs42l51 codec driver is enum, while the
current driver accesses wrongly via value.integer.value[].  They have
to be via value.enumerated.item[] instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>