commit a0e80fbd upstream.

The flash loader has been seen on a Telit UE910 modem. The flash loader
is a bit special, it presents both an ACM and CDC Data interface but
only the latter is useful. Unless a magic string is sent to the device
it will disappear and the regular modem device appears instead.
Signed-off-by: Jonas Jonsson <jonas@ludd.ltu.se>
Tested-by: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>