rt2x00: Add link tuner safe RX toggle states

This adds 2 new states which both are used to toggle
the RX. These new states are required for usage
inside the link tuner thread, because the normal
RX toggling will stop the link tuner thread.
While it is possible that the link tuner thread itself
is the caller of the RX toggle (when using software
antenna diversity).

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