IB/mlx5: Allocate flow DB only on PF IB device

A flow DB is a shared resource between PF and representors,
need to allocate it only when creating the PF IB device.
Once we add IB representors, they will use the flow db which was
created by the PF.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>