ath9k: remove mode specific default noise floor values

ath9k: remove mode specific default noise floor values

The NOISE_FLOOR array we have is mode specific, and the only
possible indexed values are A, B and G. The mode routine only
can return G or A, so this is band specific. Then since the
values for A and G (5ghz or 2ghz) are the same (-96) we simply
remove the array and use a static value.

If we later determine we want to use special values for
HT configurations we can use the new mac80211 conf_is_ht*()
helpers.

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