ath9k: Fix get channel default noise floor

Commit 8da58553 ("ath9k: Use calibrated noise floor value
when available") introduced regression in ath9k_hw_getchan_noise
where per chain nominal noise floor has been taken instead default
for channel.
Revert to original default channel noise floor.

Fixes: 8da58553

 ("ath9k: Use calibrated noise floor value when available")
Reported-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>