ASoC: omap-mcbsp: buffer size constraint only applies to playback stream

In capture stream the buffer size does not need to be constrained to be
bigger then the McBSP FIFO.
In capture the FIFO content is taken out in period length burst, this
enusres that the FIFO is not going to overflow.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Liam Girdwood <lrg@ti.com>