OMAP3: McBSP: Use the port's buffer_size when calculating tx delay

Sicne the platform data's buffer_size now holds the full size
of the FIFO, there is no longer need to handle the ports
differently.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolsfonmicro.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>