iwlwifi: allow user to set max rate

allow user to set max rate through #iwconfig <inteface> rate XXX.
mac80211 will try to force this if user set it, but driver is not
in sync which cause mac80211 to report wrong current rate. This
patch will check if max rate is set and force it in rate scaling

Signed-off-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>