nl80211: rename csa counter attributes countdown counters

We want to reuse the attributes for other counters such as BSS color
change. Rename them to more generic names.

Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20200811080107.3615705-1-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>