OMAP: McBSP: Rename thres sysfs symbols

This patch renames the symbols that handles threshold
sysfs properties. This way we can add more sysfs properties
to them.

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