IB/mlx5: Add packet based credit mode support

The device can support two credit modes, message based (default) and
packet based. In order to enable packet based mode, the QP should be
created with special flag that indicates this.

This patch adds support for the new DV QP creation flag that can be used
for RC QPs in order to change the credit mode.

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