umbim: add netifd proto handler for mbim style lte modems

tested on vodafone k5105

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42262 3c298f89-4303-0410-b956-a3cf2f4a3e73