qapi-visit: Use common idiom in gen_visit_fields_decl()

We have several instances of methods that do an early exit if
output is not needed, then log that output is being generated,
and finally produce the output; see qapi-types.py:gen_object()
and qapi-visit.py:gen_visit_implicit_struct().  The odd man
out was gen_visit_fields_decl(); rearrange it to be more like
the others.  No semantic change or difference to generated code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-12-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>