cfg80211: Add a function to iterate all BSS entries

Add a function that iterates over the BSS entries associated with a
given wiphy and calls a callback for each iterated BSS. This can be
used by drivers in various ways, e.g., to evaluate some property for
all the BSSs in the medium.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>