sfc: cleanup a condition in efx_udp_tunnel_del()

Presumably if there is an "add" function, there is also a "del"
function.  But it causes a static checker warning because it looks like
a common cut and paste bug.

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