ofproto/xlate: Move is_admissible() up

No functional changes.  This is just for better readability
when the multicast snooping learning and sending functions
are added to the code.

Acked-by: Thomas Graf <tgraf@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>