Blackfin: bf51x/bf52x: fix 16/32bit SPORT MMR helpers

The RX/TX address is always the same regardless of the size of the access.
That means there is no dedicated "16bit" or "32bit" MMR.  Trying to use
these currently leads to compile errors.  So change everything to use the
right MMR define.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>