rt2x00: Introduce 3 queue commands in drivers (start, kick, stop).

As part of the queue refactoring, we now introduce
3 queue commands: start, kick, stop.

 - Start: will enable a queue, for TX this will
   not mean anything, while for beacons and RX
   this will update the registers to enable the queue.
 - Kick: This will kick all pending frames to
   the hardware. This is needed for the TX queue
   to push all frames to the HW after the queue
   has been started
 - Stop: This will stop the queue in the hardware,
   and cancel any pending work (So this doesn't
   mean the queue is empty after a stop!).

Move all code from the drivers into the appropriate
functions, and link those calls to the old rt2x00lib
callback functions (we will fix this later when we
refactor the queue control inside rt2x00lib).

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>