pf: remove switch (af) default cases

pf_setup_pdesc() panics if address family is neither AF_INET nor
AF_INET6. So remove useless af switch defaults here and there.
Always use "switch(af)" instead of "if (af) else" for af dependent
code. Always use AF_ defines instead of PF_ when checking af values.
ok claudio mpf henning

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, fb75e2fc14
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46940