rt2x00: Make use of MAC80211_LED_TRIGGERS

Make use of the led triggers provided by mac80211 to control
the led status. This can be enabled through a per-driver
configuration option which will automatically enable the
generic handler in rt2x00lib.

This has been enabled for rt2500usb and rt73usb for the moment
since the led class will call set_brightness in irq context which
will not work correctly with the usb drivers who need to sleep.

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