iwlagn: reprogram AP STA after assoc

Instead of unconditionally sending unassoc RXON,
before any assoc RXON, re-send only the AP STA
entry which is required after the BSSID has been
programmed into the device to set up internal
filters in the microcode properly.

This fixes some issues that we correlated with
sending a lot of RXON commands to the device.

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