pf: dedupe layer 4 protocol code in pf_setup_pdesc()

In pf_setup_pdesc() the code for analysing TCP and UDP headers was
the same for v4 and v6. Deduplicate by moving the protocol switch
after the address family switch.
ok henning@ claudio@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, 72cf18cc6e
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46647