ath9k: preserve caldata history buffer across scanning

caldata's channel info is never filled with operating channel
info which is causing the operating channel's noise floor
history buffer is reset to default nf during channel change.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>