net: qmi_wwan: fixup missing ethernet header (firmware bug workaround)

A number of LTE devices from different vendors all suffer from the
same firmware bug: Most of the packets received from the device while
it is attached to a LTE network will not have an ethernet header. The
devices work as expected when attached to 2G or 3G networks, sending
an ethernet header with all packets.

This driver is not aware of which network the modem attached to, and
even if it were there are still some packet types which are always
received with the header intact.

All devices supported by this driver have severely limited
networking capabilities:
 - can only transmit IPv4, IPv6 and possibly ARP
 - can only support a single host hardware address at any time
 - will only do point-to-point communcation with the host

Because of this, we are able to reliably identify any bogus raw IP
packets by simply looking at the 4 IP version bits.  All we need to
do is to avoid 4 or 6 in the first digit of the mac address.  This
workaround ensures this, and fix up the received packets as necessary.

Given the distribution of the bug, it is believed that the source is
the chipset vendor.  The devices which are verified to be affected are:
 Huawei E392u-12 (Qualcomm MDM9200)
 Pantech UML290  (Qualcomm MDM9600)
 Novatel USB551L (Qualcomm MDM9600)
 Novatel E362    (Qualcomm MDM9600)

It is believed that the bug depend on firmware revision, which means
that possibly all devices based on the above mentioned chipset may be
affected if we consider all available firmware revisions.

The information about affected devices and versions is likely
incomplete.  As the additional overhead for packets not needing this
fixup is very small, it is considered acceptable to apply the
workaround to all devices handled by this driver.

Reported-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>