net: mhi: Add raw IP mode support

MHI net is protocol agnostic, the payload protocol depends on the modem
configuration, which can be either RMNET (IP muxing and aggregation) or
raw IP. This patch adds support for incomming IPv4/IPv6 packets, that
was previously unconditionnaly reported as RMNET packets.

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