mac80211: rename csa counters to countdown counters

We want to reuse the functions and structs 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/20200811080107.3615705-2-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>