OMAP: McBSP: Use textual values in DMA operating mode sysfs files

Use more descriptive than numerical value when showing and storing the
McBSP DMA operating mode. Show function is using similar syntax than e.g.
the led triggers so that all possible values for store function are
printed but with current value surrounded with square brackets.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Cc: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>