usbnet: Fix ordering of actions in usbnet_stop.

Make sure all software activity is quiescent (callouts and tasks,
including ifmedia and mii callbacks -- anything that might trigger
register access) before asking the driver to stop the hardware.  This
way, the driver uno_stop routine is guaranteed exclusive access to
the registers.

This will also enable us to simplify the callouts and tasks so they
don't have to check the software state -- to be done in a separate
commit.

(riastradh)