tty: serial: 8250: Cleanup p->em485 in serial8250_unregister_port

Formally, currently there is no memory leak, but if
serial8250_ports[line] is reused with other 8250 driver, then em485
will be already activated and it will cause issues.

Fixes: e490c914 ("tty: Add software emulated RS485 support for 8250")
Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>