ASoC: mmp-sspa: clear transmit phase bit for non-stereo formats

The transmit phase register value is never cleared during hw params.
So once hw params sets this bit to handle a two channel format, it
remains configured for dual-phase, which is not desirable for mono
playback.

Signed-off-by: Kyle Russell <bkylerussell@gmail.com>
Link: https://lore.kernel.org/r/20201106145905.365903-1-bkylerussell@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>