net/mlx5e: Support TSO and TX checksum offloads for GRE tunnels

Add TX offloads support for GRE tunneled packets by reporting the needed
netdev features.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>