cfg80211: rename offchannel_chain structs to background_chain to avoid confusion...

cfg80211: rename offchannel_chain structs to background_chain to avoid confusion with ETSI standard

ETSI standard defines "Offchannel CAC" as:
"Off-Channel CAC is performed by a number of non-continuous checks
spread over a period in time. This period, which is required to
determine the presence of radar signals, is defined as the Off-Channel
CAC Time..
Minimum Off-Channel CAC Time 6 minutes and Maximum Off-Channel CAC Time
4 hours..".
mac80211 implementation refers to a dedicated hw chain used for continuous
radar monitoring. Rename offchannel_* references to background_* in
order to avoid confusion with ETSI standard.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/4204cc1d648d76b44557981713231e030a3bd991.1638190762.git.lorenzo@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>