carl9170: fix bad rssi reading

Fix rssi calculation error which was introduced in otus to ar9170
porting.

Signed-off-by: Hiroaki KAWAI <hiroaki.kawai@gmail.com>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>