net: cls_u32: fix error code for invalid flags

'err' variable is not set in this test, we would return whatever
previous test set 'err' to.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>