usbd not opening all usb busses for event watching.

[Darren Pilgrim]

I appear to be running up against the hard-coded limit of four usb hubs in
usbd.  The problems were devices not attaching properly.  The machines in
question are a new notebook with four USB 2.0 ports and an older desktop
with onboard USB 1.1 and a USB 2.0 card.  The notebook produces 5 hubs and
the desktop produces 7.

The problems disappeared after I increased MAXUSBDEV to match the number of
hubs present.  This isn't really a bug, so I wasn't sure if send-pr was
appropriate.  Should I file a PR for this?