rt2x00: Fix MCU_SLEEP arguments

Legacy driver uses 0xff as the second argument for the MCU_SLEEP
command. It is still unknown what the values actually mean, but
this will at least keep the command in-sync with the original
driver.

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>