qmi_wwan: add Dell DW5811e

This is a Dell branded Sierra Wireless EM7455. It is operating in
MBIM mode by default, but can be configured to provide two QMI/RMNET
functions.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>