ASoC: davinci-mcasp: Fix TDM slot rx/tx mask associations

Fixes the associations between the tx_mask and rx_mask and the associated
playback / capture streams during setting of the TDM slot. With this
patch in place it is now possible for example to only populate tx_mask
(leaving rx_mask as 0) for output-only codecs to control the TDM slot(s)
the McASP serial port uses for transmit. Before that, this scenario
would incorrectly rely on the rx_mask for this.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>