rt2x00: Clean up rt2800usb.h.

Remove unused RXD_DESC_SIZE define and remove duplicated RXWI definitions
from rt2800.h.

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>