iwlwifi: fix searching for best rate in new search column

This patch fixes a bug in Rate Scaling. When moving from SISO to MIMO we
need to choose the lowest higher rate, instead of choosing the highest in MIMO.
No doing this can lead to a high packet loss in the highest rate in MIMO,
leading not to move MIMO although lower in MIMO could give a better TPT.

Signed-off-by: Guy Cohen <guy.cohen@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>