TTY: hvcs, use tty from tty_port

No refcounting, just a switch. The locking in the driver prevents
races, so in fact the refcounting is not needed. But while we have a
tty in tty_port, don't duplicate that and remove the one from
hvcs_struct.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>