rt2x00: Start bugging when rt2x00lib doesn't filter SW diversity

rt2x00lib should filter SW diversity out before sending any configuration
changes to the driver. When rt2x00lib fails to do this, it is important
that such events are reported because it _must_ be fixed.
So upgrading the error level to a BUG_ON() which will make sure
this bug gets noticed whenever it happens.

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