mlx4: Return EOPNOTSUPP instead of ENOTSUPP

In commit b45f0674 ("mlx4: xdp: Allow raising MTU up to one page minus eth and vlan hdrs"),
it changed EOPNOTSUPP to ENOTSUPP by mistake.  This patch fixes it.

Fixes: b45f0674 ("mlx4: xdp: Allow raising MTU up to one page minus eth and vlan hdrs")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>