mlx4: rename conflicting netdev_priv() to mlx4_netdev_priv()

netdev_priv() is a LinuxKPI function which was used with the old ifnet
linux/netdevice.h implementation which was not adaptable to modern
Linux drviers unless rewriting them for ifnet in first place which
defeats the purpose.
Rename the netdev_priv() calls in mlx4 to mlx4_netdev_priv()
returning the ifnet softc to avoid conflicting symbol names
with different implementations in the future.

MFC after: 3 days
Reviewed by: hselasky, kib
Differential Revision: https://reviews.freebsd.org/D32640