ath9k: make request to get the noisefloor threshold band specific

Lets make the request to get the current noise floor threshold
from the EEPROM band specific as it is band specific, not mode
specific.

This also adds a backpointer on the private channel structure
back to the ieee80211_channel structure as this is now needed during
ath9k_hw_getnf().

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>