net/mlx5e: Increase Striding RQ minimum size limit to 4 multi-packet WQEs

This is to prevent the case of working with a single MPWQE
(1 WQE is always reserved as RQ is linked-list).
When the WQE is fully consumed, HW should still have available buffer
in order not to drop packets.

Fixes: 461017cb

 ("net/mlx5e: Support RX multi-packet WQE (Striding RQ)")
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Cc: kernel-team@fb.com
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>