Blackfin Serial Driver: handle anomaly 05000231

05000231 - UART STB Bit Incorrectly Affects Receiver Setting
For processors affected by this, we cannot safely allow CSTOPB to be set
as the UART will then be unable to properly clock in bytes.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>