staging: gdm724x: gdm_lte: Constify gdm_netdev_ops

Fix the following checkpatch.pl warning:
WARNING: struct net_device_ops should normally be const
+static struct net_device_ops gdm_netdev_ops = {

Signed-off-by: Imre Deak <imre.deak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>