OMAP: McBSP: Swap CLKS source definition

This is just a readability and debugging improvement. As selection bit in
DEVCONF register is cleared when using 96 MHz PRCM source and set when using
external CLKS pin, change definitions to be sync with these.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>