average: provide macro to create static EWMA

Having the EWMA parameters stored in the runtime struct imposes
memory requirements for the constant values that could just be
inlined in the code. This particularly makes sense if there are
a lot of such structs, for example in mac80211 in the station
table where each station has a number of these in an array, and
there can be many stations.

Provide a macro DECLARE_EWMA() that declares the necessary struct
and inline functions to access it with the parameters hard-coded;
using this also means the user no longer needs to 'select AVERAGE'
as it's entirely self-contained.

In the mac80211 case, on x86-64, this actually slightly *reduces*
code size, while also saving 80 bytes of runtime memory per sta.

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