cfg80211: allow drivers to support random MAC addresses for scan

Add the necessary feature flags and a scan flag to support using
random MAC addresses for scan while unassociated.

The configuration for this supports an arbitrary MAC address
value and mask, so that any kind of configuration (e.g. fixed
OUI or full 46-bit random) can be requested. Full 46-bit random
is the default when no other configuration is passed.

Also add a small helper function to use the addr/mask correctly.

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