ASoC: davinci-mcasp: Fix master capture only mode

When McASP is used as TX/RX synchronous (TX side generating clocks for RX
side also) and only capture is used we need to configure the number of TX
slots in order McASP to be able to generate the Frame sync.

Fixes: 9273de1940d9e ("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>