net/mlx5e: Refactor retrival of skb from rx completion element (cqe)

Factor the relevant code into a static inline helper (skb_from_cqe)
doing that.

Move the call to napi_gro_receive to be carried out just
after mlx5e_complete_rx_cqe returns.

Both changes are to be used for the VF representor as well
in the next commit.

This patch doesn't change any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>