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>