Sporadically the network over gem(4) interface hang on sparc64 and

Sporadically the network over gem(4) interface hang on sparc64 and
macppc.  Receiving packets stopped, ifconfig down/up made it work
again.
In the tick timeout handler refill the receive ring if it is empty.
The logic is taken from hme(4).  Also protect the register access
and ifp counters with splnet().
In gem_rx_watchdog() is a workaround for a hardware bug.  It resets
the hardware when there is no progress.  If the fifo pointers
advanced a bit, it got stuck anyway.  So restart the receive watchdog
timeout in that case.
OK mpi@