rt2x00: use DECLARE_EWMA

Instead of using the out-of-line EWMA calculation, use DECLARE_EWMA()
to create static inlines. On x86/64 this results in code that's one
byte larger (for me), but reduces struct link_ant and struct link
size by the two unsigned long values that store the parameters each.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>