Staging: epl: fix netdev->priv b0rkage

netdev->priv is now gone, use netdev_priv() instead.
This fixes the build error in the network driver within the epl stack.

Cc: Daniel Krueger <daniel.krueger@systec-electronic.com>
Cc: Ronald Sieber <Ronald.Sieber@systec-electronic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>