IB/mlx5: Save flow table priority handler instead of index

Saving the flow table priority object's pointer in the flow handle
is necessary for downstream patches since the sniffer flow table isn't
placed at the standard flow_db structure but in a different database.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>