ASoC: omap-mcbsp: When closing the port select PRCM source for CLKS signal

If external source for the CLKS signal selection kept after the port is no
longer in use the system might refuse to go suspend.
There is also a chance that the external clock is not running when next
time the McBSP port is started which can result errors when we try to
access McBSP registers.
Reset the CLKS source back to PRCM source unconditionally.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Liam Girdwood <lrg@ti.com>