net: mhi: Add mbim proto

net: mhi: Add mbim proto

MBIM has initially been specified by USB-IF for transporting data (IP)
between a modem and a host over USB. However some modern modems also
support MBIM over PCIe (via MHI). In the same way as QMAP(rmnet), it
allows to aggregate IP packets and to perform context multiplexing.

This change adds minimal MBIM data transport support to MHI, allowing
to support MBIM only modems. MBIM being based on USB NCM, it reuses
and copy some helpers/functions from the USB stack (cdc-ncm, cdc-mbim).

Note that is a subset of the CDC-MBIM specification, supporting only
transport of network data (IP), there is no support for DSS. Moreover
the multi-session (for multi-pdn) is not supported in this initial
version, but will be added latter, and aligned with the cdc-mbim
solution (VLAN tags).

This code has been inspired from the mhi_mbim downstream implementation
(Carl Yin <carl.yin@quectel.com>).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>