iwlagn: fix RXON issues

The RXON rework resulted in a massive loss of
throughput because we weren't programming the
device completely correctly -- the BSSID has
to be programmed into the device before the
AP station is uploaded. To fix this, simply
always send the unassoc RXON, i.e. even when
it was already unassoc so that the BSSID and
some other parameters are updated properly.

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