IB/mlx5: Support extended number of strides for Striding RQ

Extends the minimum single WQE strides from 64 to 8, which is exposed
by the "min_single_wqe_log_num_of_strides" field of striding_rq_caps.
Choose right number of strides based on FW capability.

Link: https://lore.kernel.org/r/20191115154555.247856-1-leon@kernel.org


Signed-off-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>