ath9k_hw: fix periodic noise floor calibration on AR9003

The periodic noise floor calibration is broken on this chip family, because
it keeps triggering a software-filtered noise floor calibration, but never
reads the result before uploading the history buffer value to the hardware.

Fix this with a call to ath9k_hw_getnf(), just like on AR9002.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>