niu: fix error handling in niu_class_to_ethflow()

There is a discrepancy here because the niu_class_to_ethflow() returns
zero on failure and one on success but the caller expected zero on
success and negative on failure.

The problem means that we allow the user to pass classes and flow_types
which we don't want.  I've looked at it a bit and I don't see it as a
very serious bug.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>