TTY: serial, define uart_port_activate/shutdown

This is a preparation for the next patches which will move the stuff
from uart_open and uart_close/hangup here. Then we will use
tty_port_* helpers.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>