mac80211: use DECLARE_EWMA

mac80211: use DECLARE_EWMA

Instead of using the out-of-line average calculation, use the new
DECLARE_EWMA() macro to declare a signal EWMA, and use that.

This actually *reduces* the code size slightly (on x86-64) while
also reducing the station info size by 80 bytes.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>