net/mlx5: Expose packet based credit mode

Packet based credit mode bit determines whether the credit mode
is done per message or packet. Expose the QP creation flag and
the HCA capability.

Signed-off-by: Danit Goldberg <danitg@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>