ASoC: davinci-mcasp: Only disable inactive serializer

As a side effect of the following commit, the active TX
serializer may get disabled which may result in distorted
audio output.

ASoC: davinci-mcasp: Add support for multichannel playback
(2952b27e)

For example, if a 4 channel I2S playback with two TX serializers
is activated. Later on, if a recording of 2 channels, with only 1 RX
serializer is started, which will also disable one of the TX
serializer because max_active_serializers is only calculated for
RX (recording) stream. This patch fixes this issue.

Signed-off-by: Vishal Thanki <vishalthanki@gmail.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>