mac80211: extend set_coverage_class signature

Extend mac80211 set_coverage_class API in order to enable ACK timeout
estimation algorithm (dynack) passing coverage class equals to -1
to lower drivers. Synchronize set_coverage_class routine signature with
mac80211 function pointer for p54, ath9k, ath9k_htc and ath5k drivers.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/ath/ath5k/mac80211-ops.c

@@ -704,7 +704,7 @@ ath5k_get_survey(struct ieee80211_hw *hw, int idx, struct survey_info *survey)
  * reset.
  */
 static void
-ath5k_set_coverage_class(struct ieee80211_hw *hw, u8 coverage_class)
+ath5k_set_coverage_class(struct ieee80211_hw *hw, s16 coverage_class)
 {
 	struct ath5k_hw *ah = hw->priv;