Revert "net/mlx5e: Block offload of outer header csum for GRE tunnel"

This reverts commit 54e1217b.

Although the NIC doesn't support offload of outer header CSUM, using
gso_partial_features allows offloading the tunnel's segmentation. The
driver relies on the stack CSUM calculation of the outer header. For
this, NETIF_F_GSO_GRE_CSUM must be a member of the device's features.

Fixes: 54e1217b ("net/mlx5e: Block offload of outer header csum for GRE tunnel")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>