This fixes a long standing bug in mxge(4) where "ifconfig mxge0 $IP"
did not bring the interface into a RUNNING state, like it does on
most (all?) other FreeBSD NIC drivers.
Thanks to gnn for mentioning the bug, and yongari for pointing out that
ether_ioctl() invokes ifp->if_init() in SIOCSIFADDR.
MFC after: 7 days