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
MFC-after: 2 weeks
Reviewed-by: hselasky, kib
X-D-R: D29366
Differential Revision: https://reviews.freebsd.org/D29497