pf: remove pointless NULL check

pfi_kkif_attach() always returns non-NULL, and we dereference the
pointer before we check it, so that's pointless.

Reported by: Coverity (CID 1007345)
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit ed6287c14168de409c5f333bda59896c8109eb70)