USB: cxacru: ADSL state management

The device has commands to start/stop the ADSL function, so this adds a
sysfs attribute to allow it to be started/stopped/restarted.  It also stops
polling the device for status when the ADSL function is disabled.

There are no problems with sending multiple start or stop commands, even
with a fast loop of them the device still works.  There is no need to
protect the restart process from further user actions while it's waiting
for 1.5s.
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Duncan Sands <duncan.sands@math.u-psud.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>