serial: bfin_sport_uart: protect changes to uart_port

Common serial API says we need to grab the port lock before modifying
the port state to prevent inconsistent state between threads.

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>