net: mhi: Set wwan device type

The 'wwan' devtype is meant for devices that require additional
configuration to be used, like WWAN specific APN setup over AT/QMI
commands, rmnet link creation, etc. This is the case for MHI (Modem
host Interface) netdev which targets modem/WWAN endpoints.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Link: https://lore.kernel.org/r/1611328554-1414-1-git-send-email-loic.poulain@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>