net: qmi_wwan: support "raw IP" mode

QMI wwan devices have traditionally emulated ethernet devices
by default. But they have always had the capability of operating
without any L2 header at all, transmitting and receiving "raw"
IP packets over the USB link.  This firmware feature used to be
configurable through the QMI management protocol.

Traditionally there was no way to verify the firmware mode
without attempting to change it.  And the firmware would often
disallow changes anyway, i.e. due to a session already being
established.  In some cases, this could be a hidden firmware
internal session, completely outside host control.  For these
reasons, sticking with the "well known" default mode was safest.

But newer generations of QMI hardware and firmware have moved
towards defaulting to "raw IP" mode instead, followed by an
increasing number of bugs in the already buggy "802.3" firmware
implementation. At the same time, the QMI management protocol
gained the ability to detect the current mode.  This has enabled
the userspace QMI management application to verify the current
firmware mode without trying to modify it.

Following this development, the latest QMI hardware and firmware
(the MDM9x30 generation) has dropped support for "802.3" mode
entirely. Support for "raw IP" framing in the driver is therefore
necessary for these devices, and to a certain degree to work
around problems with the previous generation,

This patch adds support for "raw IP" framing for QMI devices,
changing the netdev from an ethernet device to an ARPHRD_NONE
p-t-p device when "raw IP" framing is enabled.

The firmware setup is fully delegated to the QMI userspace
management application, through simple tunneling of the QMI
protocol. The driver will therefore not know which mode has been
"negotiated" between firmware and userspace. Allowing userspace
to inform the driver of the result through a sysfs switch is
considered a better alternative than to change the well established
clean delegation of firmware management to userspace.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>