[PATCH] ipw2200: Exponential averaging for signal and noise Level

This patch replaces sliding averaging by exponential averaging for
reporting the wireless statistics for signal and noise level for ipw2200.
See details from: http://www.ces.clemson.edu/linux/ipw2200_averages.shtml

Signed-off-by: Bill Moss <bmoss@clemson.edu>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>