mac80211: refactor monitor representation in sdata

Insert the u32 monitor flags variable in a new structure
that represents a monitor interface.
This will allow to add more configuration variables to
that structure which will happen in an upcoming change.

Signed-off-by: Aviya Erenfeld <aviya.erenfeld@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>