rt2x00: Fix quality houskeeping for software diversity

Antanna quality statistics is not handled correctly, which leads to
software diversity being shutdown completly.

The main problem is that during antenna diversity statistics can
be reset resulting in loosing the signal strength just before
evaluation.

rssi history is not updated correctly leading to incorrect comparison
material for basing antenna switching on.

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