Add support for Telit LN910Cx 0x1250 composition

0x1250: rmnet, tty, tty, tty, tty
Signed-off-by: Carlo Lobrano <c.lobrano@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>