ASoC: omap: N810: Don't select CONFIG_OMAP_MUX but make it as dependency

Not all omap boards use kernel based pin multiplexing so
CONFIG_SND_OMAP_SOC_N810 should not select it by default as it can make
harm to other boards in multi-board kernels.

Therefore put CONFIG_OMAP_MUX as a dependency to N810 ASoC machine driver.

Thanks to Tony Lindgren <tony@atomide.com> for noticing.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>