iwlagn: fix agn rate scaling

    Rate scaling in agn can be broken because of these two problems
    1- the LQ command could not sent, we try to call LQ command before
       post association will be rejected, this will cause the driver to
       break in rs_tx_status reporting initial rate does not match.
    2- In rs_rate_init we can get a value of 12, rate 60, for
       last_txrate_idx, this rate only valid in SISO/MIMO mode only

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>