ASoC: blackfin: bf5xx-sport: Allow setting rx and tx mask independently

Since the hardware supports it there is no need to artificially limit this to
just being able to set the same mask for both tx and rx.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>