Adding support for the MBIM mode in some Sierra Wireless devices.

Some Sierra Wireless firmwares support CDC MBIM but have no CDC
Union funtional descriptor. This violates the MBIM specification,
but we can easily work around the bug by looking at the Interface
Association Descriptor instead.  This is most likely what
Windows uses too, which explains how the firmware bug has gone
unnoticed until now.

This change will not affect any currently supported device
conforming to the NCM or MBIM specifications, as they must have
the CDC Union descriptor.

Cc: Greg Suarez <gsuarez@smithmicro.com>
Cc: Alexey Orishko <alexey.orishko@stericsson.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>