sfc: fix an off by one bug

This bug is harmless because it's just a sanity check and we always
pass valid values for "encap_type" but the test is off by one.

Fixes: 9b410801

 ("sfc: insert catch-all filters for encapsulated traffic")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>