net: cdc_ether: allow combined control and data interface

Some Icera based Huawei modems handled by this driver are not
completely CDC ECM compliant, using the same USB interface for both
control and data. The CDC functional descriptors include a Union
naming this interface as both master and slave, so it is supportable
by relaxing the descriptor parsing in case these interfaces are
identical.

This has been tested on a Huawei K3806 and verified to add support
for that device.

Reported-and-tested-by: Enrico Mioso <mrkiko.rs@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: David S. Miller <davem@davemloft.net>