mlx5: remove dependency on ifnet specifics of linux/netdevice.h

Rename the last remaining bits depending on ifnet from linux/netdevice.h
instead of using the compat macros. This helps clearing up
struct netdevice being struct ifnet from linux/netdevice.h.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky, kib
Differential Revision: https://reviews.freebsd.org/D29497

(cherry picked from commit 4ded022d3b8948e92c8f8353364ff2c79d720e68)