net/mlx5e: Fix wrong calculation of header index in HW_GRO

The HW doesn't wrap the CQE.shampo.header_index field according to the
headers buffer size, instead it always increases it until reaching overflow
of u16 size.

Thus the mlx5e_handle_rx_cqe_mpwrq_shampo handler should mask the
CQE header_index field to find the actual header index in the headers buffer.

Fixes: f97d5c2a ("net/mlx5e: Add handle SHAMPO cqe support")
Signed-off-by: Khalid Manaa <khalidm@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>