rt2x00: conf_tx() only need register access for WMM queues

conf_tx() in rt61pci and rt73usb only have to check once
if the queue_idx indicates a non-WMM queue and break of
the function immediately if that is the case.

Only the WMM queues need to have the TX configuration written
to the registers.

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