net/mlx5e: Don't try to modify CQ moderation if it is not supported

net/mlx5e: Don't try to modify CQ moderation if it is not supported

BugLink: http://bugs.launchpad.net/bugs/1557950
If CQ moderation is not supported by the device, print a warning on
netdevice load, and return error when trying to modify/query cq
moderation via ethtool.

Fixes: f62b8bb8f2d3 ('net/mlx5: Extend mlx5_core to support ConnectX-4
Ethernet functionality')
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from linux-next commit 7524a5d88b94afef8397a79f1e664af5b7052c22)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>