OMAP: McBSP: Let element DMA mode hit retention also

The device no longer hits retention if element DMA
mode is taken for at least the duration of the
serial console timeout. Force element DMA mode to
shut down through smartidle.

Signed-off-by: Eero Nurkkala <ext-eero.nurkkala@nokia.com>
Acked-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>