iwl3945 : fix rate scaling

Patch fixes the bug 1900 at
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1900

Issues:
Throughput and success ratio calculations were not done properly.
Number of retries were exceeding 16.

Fix:
Patch fixes above issues by doing window calculations inline with iwlwifi
Patch adds sanity check to limit number of retries to 16.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Acked-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>