ASoC: ti: davinci-mcasp: Fix slot mask settings when using multiple AXRs

If multiple serializers are connected in the system and the number of
channels will need to use more than one serializer the mask to enable the
serializers were left to 0 if tdm_mask is provided

Fixes: dd55ff83 ("ASoC: davinci-mcasp: Add set_tdm_slots() support")

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