nl80211: allow using wdev identifiers to get scan results

Most dump callbacks, including the scan results one, use
the netdev to identify what to do, which is incorrect for
the P2P_DEVICE support, it needs to be able to get the
scan result from the wdev. Change all dumps to unify the
code, but ones other than scan don't really support being
executed on a wdev that has no netdev.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>