davinci-mcasp: use bitfield definitions for PDIR

The current driver creates value for set/clr of PDIR using (x<<26) instead
of the #defines that are convieniently made available.

Update the driver to use the bitfield definitions of PDIR. There is no
functional change introduced by this patch.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Reviewed-by: James Nuss <jamesnuss@nanometrics.ca>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>