net/mlx5: Report multi packet WQE capabilities

Multi packet WQE enables sending multiple fix sized packets
using a single WQE. The exposed field reports such HW support.

Signed-off-by: Bodong Wang <bodong@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>