iwlwifi: support explicit monitor interface

Support explicit monitor interface to keep
injection working after the HW queue changes.
This also finally enables sniffer mode.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>