iwlagn: fix agn rate scaling

Sometime Tx reply rate different than what rate scale expecting
causing rate scale to bail out. This could cause failing to
commit LQ cmd. This patch will try to solve this instead of just
bail out. It also make sure we start with a valid rate.

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>