serial: 8250: lock port for stop_rx() in omap8250_irq()

The uarts_ops stop_rx() callback expects that the port->lock is
taken and interrupts are disabled.

Fixes: 1fe0e1fa ("serial: 8250_omap: Handle optional overrun-throttle-ms property")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230525093159.223817-4-john.ogness@linutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>