iwlwifi: fix in-column rate scaling

This patch fixes cases that the code raised or didn't decrease
the rate although the success ratio was not good.

Signed-off-by: Guy Cohen <guy.cohen@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>