iwlwifi: add test to determine if interface in monitor mode

mac80211 sets driver in monitor mode through configuring the
RX filters. We cannot trust priv->iw_mode to be accurate
regarding monitor mode as iw_mode is only set in add_interface,
which is not called by mac80211 when in monitor mode.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>