serial: Take care starting a hung-up tty's port

tty_port_hangup sets a port's tty field to NULL (holding the port lock),
but uart_tx_stopped, called from __uart_start (with the port lock),
uses the tty field without checking for NULL.

Change uart_tx_stopped to treat a NULL tty field as another stopped
indication.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>