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>