P2P: Cleanup handling of unknown peer in PD Request processing

P2P: Cleanup handling of unknown peer in PD Request processing

If a Provision Discovery Request is received for an unknown peer, a new
device entry is being added, but the flow continues without updating the
local p2p_device pointer, requiring to check the pointer value before
every access.

1. Change this, so once a device is added, the flow updates the local
   p2p_device pointer and avoids the checks later in the flow.
2. If the device is not known even after adding it, skip the processing,
   send the PD Response, and return.

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