RDMA/mlx5: Add support to multiple priorities for FDB rules

Currently, the driver ignores the user's priority for flow steering
rules in FDB namespace. Change it and create the rule in the right
priority.
It will allow to create FDB steering rules in up to 16 different
priorities.

Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>