sfc: fix an off-by-one compare on an array size

encap_type should be checked to see if it is greater or equal to
the size of array map to fix an off-by-one array size check. This
fixes an array overrun read as detected by static analysis by
CoverityScan, CID#1398883 ("Out-of-bounds-read")

Fixes: 9b410801 ("sfc: insert catch-all filters for encapsulated traffic")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>