ASoC: omap-mcbsp: Fix compilation error due to leftover code

Part of commit (which patches sound/soc/omap/mcbsp.c file):
8fef626 ARM/ASoC: omap-mcbsp: Remove CLKR/FSR mux configuration code

since the tree where it has been applied did not had the earlier patch:
d0db84e ASoC: omap-mcbsp: Fix 6pin mux configuration
which changed code around omap_mcbsp_6pin_src_mux().

Because of the missing part from 8fef626 the sound/soc/omap/mcbsp.c does
not compile in linux-next.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>