backports: remove AVERAGE, backport DECLARE_EWMA

My patches to remove the out-of-line EWMA implemenation (under
Kconfig symbol AVERAGE) went into net-next, so now we only need
to backport DECLARE_EWMA as the only necessary macro.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>