iw: add auto parameter to set distance command

iw: add auto parameter to set distance command

Add auto parameter to set distance command in order to enable ACK timeout
estimation algorithm (dynack). Dynack is automatically disabled setting valid
value for coverage class. Currently dynack is supported just by ath9k

This patch is based on "configure dynack through mac80211/cfg80211 stack"
patchset sent on linux-wireless

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