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.

(cherry picked from commit 9d593d5a76f22a315d7c6ab99e5e76003ec2b3f5)