iwlagn: update QoS before commit associated RXON

RXON command without association bit can clear the QoS info in the
uCode. Therefore, before sending the associated RXON, we need to
send the QoS command just in case.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>