mlx4: replace LinuxKPI macros with ifnet functions

The LinuxKPI net_device actually is an ifnet; in order to further
clean that up so we can extend "net_device" replace the few macros
inline in mlx4.

Sponsored by: The FreeBSD Foundation
MFC after: 12 days
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D30476