rt2x00: Move generic TX frame writing code into rt2x00queue

The write_tx_data functions in rt2x00pci and rt2x00usb have
a lot in common. This moves that duplicate code into
rt2x00queue_write_tx_frame().

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