ath9k_hw: fix noisefloor timeout handling on AR9003

When the noisefloor calibration times out, do not load -50 into
the registers, since this might cause rx issues. Instead, leave
enough time for the noise floor calibration to complete until
the next check.

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