Small fix in the DSI tickle handling

Due to a regression introduced in 9355ebcc5744bbd445a0c8cb8b0a0ca201d5499a
the server may send an extra DSI tickle immediately after a "volume modified"
DSI attention notification. This will only happen if the connection is

o completely idle, and
o the volume was modified by another AFP client

The result is an extra DSI tickle every 30 seconds.

Fixes bug #528.