rt2x00: Fix failed SLEEP->AWAKE and AWAKE->SLEEP transitions.

(Based on a patch created by Ondrej Zary)

In some circumstances the Ralink devices do not properly go to sleep
or wake up, with timeouts occurring.
Fix this by retrying telling the device that it has to wake up or
sleep.

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