Fixup for f943348d - caused deadlock.
Fixes the exit condition of the mailbox FSMs -- previously they returned
whether they were sending a datagram or not, and the parent assumed that
this meant they were done if they didn't want to send a datagram. Since
earlier patches can cause idle cycles now, this could cause unexpected
early exit, so they now return whether they're complete or not
explicitly.
Gavin Lambert
Florian Pose