iwlwifi: clean up unused NL80211_IFTYPE_MONITOR for Monitor mode

This patch clean up the code for NL80211_IFTYPE_MONITOR mode,
priv->iw_mode is set in add_interface, but add_interface is never called
for monitor mode.
The only way mac80211 informs us about monitor mode is through
configuring filter; since iw_mode will never set to
NL80211_IFTYPE_MONITOR, modify and remove all the code refer to
NL80211_IFTYPE_MONITOR and replace with iwl_is_monitor_mode() function
call.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>