serial: bfin_5xx: grab port lock before making port termios changes

The port lock exists to protect these resources, so we need to grab it
before making changes.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>