mlx4/OFED: replace the struct net_device with struct ifnet

Given all the code does operate on struct ifnet, the last step in this
longer series of changes now is to rename struct net_device to
struct ifnet (that is what it was defined to in the LinuxKPi code).
While mlx4 and OFED are "shared" code the decision was made years ago
to not write it based on the netdevice KPI but the native ifnet KPI
for most of it. This commit simply spells this out and with that
frees "struct netdevice" to be re-done on LinuxKPI to become a more
native/mixed implementation over time as needed by, e.g., wireless
drivers.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30515