ath5k: Use EWMA factor of 1024 instead of 1000

This prepares the only place which uses the EWMA library so far for the
performance improved implementation coming up, which requires factor and
weight to be a power of two.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>