P2P: Save group common frequencies in invitation result

P2P: Save group common frequencies in invitation result

Save the group common frequencies when starting a GO due to
an invitation signaling requesting to re-invoke a persistent GO.

To do so, move the code that handles the translation of p2p_channels to
frequency list into a public function so it can be re-used both when GO
Negotiation is done and invitation signaling is done.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>