ASoC: mchp-spdiftx: remove 'TX' from playback stream name

Do not include the 'TX' in the stream name since it's obvious for
playback.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Link: https://lore.kernel.org/r/20201009123527.2770629-1-codrin.ciubotariu@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>