rt2x00: Add watchdog functions for HW queue

Add watchdog functions for managing the Queues inside the hardware.
Normally the driver doesn't have much to do with these queues
directly, but the Ralink drivers did implement watchdog functions
for these. These watchdog functions are not triggered that often,
compared to the other watchdog functions, but I have at least
seen them trigger once or twice during a long stresstest run.

v2: Add extra documentation for register fields

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>