LinuxKPI/OFED/mlx4: cleanup netdevice.h some more

This removes all unused bits from linux/netdevice.h and migrates two
inline functions into the mlx4 and ofed code respectively.

This gets the mlx4/ofed (struct ifnet) specific bits down to 7 lines
in netdevice.h.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky, kib
Differential Revision: https://reviews.freebsd.org/D30461

(cherry picked from commit c35034b338eb4de6fb42fd855865bce21c81a225)