rt2x00: Support hardware RTS and CTS-to-self frames

If the driver has set the set_rts_threshold() callback function
to mac80211 it is capable of generating RTS and CTS-to-self frames
inside the hardware and rt2x00lib doesn't have to create them
in software.
Only rt2800pci and rt2800usb support this feature.

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