net: cdc_mbim: handle unaccelerated VLAN tagged frames

This driver maps 802.1q VLANs to MBIM sessions. The mapping is based on
a bogus assumption that all tagged frames will use the acceleration API
because we enable NETIF_F_HW_VLAN_CTAG_TX. This fails for e.g. frames
tagged in userspace using packet sockets. Such frames will erroneously
be considered as untagged and silently dropped based on not being IP.

Fix by falling back to looking into the ethernet header for a tag if no
accelerated tag was found.

Fixes: a82c7ce5 ("net: cdc_ncm: map MBIM IPS SessionID to VLAN ID")
Cc: Greg Suarez <gsuarez@smithmicro.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>