ASoC: omap: Remove OMAP_MUX dependency from Nokia N810 audio support

Commit e9f5f1e4 ("ARM: OMAP2+: Remove legacy mux code") removed
CONFIG_OMAP_MUX making impossible to build Nokia N810 audio support. Remove
this dependency so we can do at least build tests.

Fixes: e9f5f1e4 ("ARM: OMAP2+: Remove legacy mux code")
Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>