net: mhi: Add protocol support

MHI can transport different protocols, some are handled at upper level,
like IP and QMAP(rmnet/netlink), but others will need to be inside MHI
net driver, like mbim. This change adds support for protocol rx and
tx_fixup callbacks registration, that can be used to encode/decode the
targeted protocol.

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