iwlwifi: Fix uCode error on association

The problem is that priv->assoc_id is set when assoc. resp frame is
received. But, when it is set, LQ cmd is still not sent to the uCode, it is
done from bg_post_assoc, which is called through a workqueue.

On the other hand, when a tx arrives at the moment when this flag is set,
but LQ is still not sent, the if condition in tx_skb will not hold and
the frame will not be dropped.  Thus, it will be sent through
which is still not in the sta table in the uCoded.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>