cfg80211: enable dynack through nl80211

Enable ACK timeout estimation algorithm (dynack) using mac80211
set_coverage_class API. Dynack is activated passing coverage class equals to -1
to lower drivers and it is automatically disabled setting valid value for
coverage class.
Define NL80211_ATTR_WIPHY_DYN_ACK flag attribute to enable dynack from
userspace. In order to activate dynack NL80211_FEATURE_ACKTO_ESTIMATION feature
flag must be set by lower drivers to indicate dynack capability.

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