tty: serial: linflexuart: improve locking in set_termios

commit 3ae3bb0f8a57c6c900baf5cbf11c3226589b8d97 from
https://github.com/nxp-auto-linux/linux



Take the port->lock when set_termios is called, otherwise if characters
are sent while IP is in init mode, the IP will hang in an uncertain state.

Issue: ALB-9101

Signed-off-by: Radu Pirea <radu-nicolae.pirea@nxp.com>
Signed-off-by: Zhantao Tang <zhantao.tang@windriver.com>