nl80211: support scan frequencies in KHz

If the driver advertises NL80211_EXT_FEATURE_SCAN_FREQ_KHZ
userspace can omit NL80211_ATTR_SCAN_FREQUENCIES in favor
of an NL80211_ATTR_SCAN_FREQ_KHZ. To get scan results in
KHz userspace must also set the
NL80211_SCAN_FLAG_FREQ_KHZ.

This lets nl80211 remain compatible with older userspaces
while not requring and sending redundant (and potentially
incorrect) scan frequency sets.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200430172554.18383-4-thomas@adapt-ip.com


[use just nla_nest_start() (not _noflag) for NL80211_ATTR_SCAN_FREQ_KHZ]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>