iw: Add support for NL80211_ATTR_WIPHY_COVERAGE_CLASS
New nl80211 attribute NL80211_ATTR_WIPHY_COVERAGE_CLASS allows setting
IEEE 802.11 coverage class, which is then used by drivers to calculate
slot time and ACK timeout for long distance links.
Two iw parameters are added: 'coverage' sets the coverage class
directly, while 'distance' is more user-friendly, as it allows to set
just the link distance and let iw do the necessary calculation itself.