iwlwifi: fix priv->iw_mode setting when multiple vif are configured

mac80211 supports multiple virtual interfaces for a single device. For
example, a managed interface (wlan0) and a monitor interface (mon0) can
exist at the same time. Thus priv->iw_mode is not sufficient to track
the wireless mode any more. The patch redefines priv->iw_mode as the
first interface mode (the same as priv->vif->type if priv->vif != NULL).
If another monitor type interface is created later, we don't change
priv->iw_mode into monitor. This way, the original interface still
works. The patch also requests mac80211 to do reassociation after we
change the Rx filter flags.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>