rt2x00: Fix antenna diversity

Fix 2 issues in antenna diversity selection.

1) the following statement will always return true.
	if ((rssi_curr - rssi_old) > -5 || (rssi_curr - rssi_old) < 5)
It is cleaner to check if the absolute value is smaller then 5.

2) Only enable software diversity when default antenna setup
indicates support for it. Don't select it when the hardware
does not indicate support for it...

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