rt2x00: Fix beaconing on rt2800.

According to the Ralink vendor driver for rt2800 we don't need a full
TXD for a beacon but just a TXWI in front of the actual beacon.
Fix the rt2800pci and rt2800usb beaconing code accordingly.

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>