Blackfin serial driver: implement support for ignoring parity/break errors

properly setting up and respecting the read_status_mask / ignore_status_mask fields of the serial core

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>