net/mlx5e: RX, Increase WQE bulk size for legacy rq

Deferred page release was added to legacy rq but its desired effect
(driver releases last fragment to page pool cache) is not yet visible
due to the WQE bulks being too small.

This patch increases the WQE bulk size to span 512 KB and clip it to
one quarter of the rx queue size.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>