IB/mlx5: Enable TX on a DEVX flow table

Flow table can be passed as a DEVX object which is a valid destination in
an EGRESS flow. Fix the original code to allow that.

Fixes: a7ee18bd

 ("RDMA/mlx5: Allow creating a matcher for a NIC TX flow table")
Signed-off-by: Alex Vesker <valex@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>