mac80211: add IEEE80211_HW_P2P_DEV_ADDR_FOR_INTF

Some devices like the current iwlwifi implementation
require that the P2P interface address match the P2P
Device address (only one P2P interface is supported.)
Add the HW flag IEEE80211_HW_P2P_DEV_ADDR_FOR_INTF
that allows drivers to request that P2P Interfaces
added while a P2P Device is active get the same MAC
address by default.

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