nl80211: add KHz frequency offset for most wifi commands

cfg80211 recently gained the ability to understand a
frequency offset component in KHz. Expose this in nl80211
through the new attributes NL80211_ATTR_WIPHY_FREQ_OFFSET,
NL80211_FREQUENCY_ATTR_OFFSET,
NL80211_ATTR_CENTER_FREQ1_OFFSET, and
NL80211_BSS_FREQUENCY_OFFSET.

These add support to send and receive a KHz offset
component with the following NL80211 commands:

- NL80211_CMD_FRAME
- NL80211_CMD_GET_SCAN
- NL80211_CMD_AUTHENTICATE
- NL80211_CMD_ASSOCIATE
- NL80211_CMD_CONNECT

Along with any other command which takes a chandef, ie:

- NL80211_CMD_SET_CHANNEL
- NL80211_CMD_SET_WIPHY
- NL80211_CMD_START_AP
- NL80211_CMD_RADAR_DETECT
- NL80211_CMD_NOTIFY_RADAR
- NL80211_CMD_CHANNEL_SWITCH
- NL80211_JOIN_IBSS
- NL80211_CMD_REMAIN_ON_CHANNEL
- NL80211_CMD_JOIN_OCB
- NL80211_CMD_JOIN_MESH
- NL80211_CMD_TDLS_CHANNEL_SWITCH

If the driver advertises a band containing channels with
frequency offset, it must also verify support for
frequency offset channels in its cfg80211 ops, or return
an error.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200430172554.18383-3-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
