tty: metag_da: avoid getting tty kref in dashtty_timer()

Getting the tty kref in dashtty_timer() is no longer necessary since it
isn't needed in fetch_data() any longer (due to changes which make the
tty flip functions refer to tty_ports instead of tty_structs), so just
pass around a channel number instead.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>