net/mlx5: fix missing assignment of variable err

The error return from a call to mlx5_flow_namespace_set_peer is not
being assigned to variable err and hence the error check following
the call is currently not working.  Fix this by assigning ret as
intended.

Addresses-Coverity: ("Logically dead code")
Fixes: 8463daf1

 ("net/mlx5: Add support to use SMFS in switchdev mode")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>