ASoC: mediatek: btcvsd fix tx stream assign

Fix tx/rx stream assign in write.
Write should use tx instead of rx.

Signed-off-by: Lumi Lee <lumi.lee@mediatek.com>
Link: https://lore.kernel.org/r/1606705875-1940-1-git-send-email-Lumi.Lee@mediatek.com


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