bluetooth: mesh: Do not skip PDU forwarding on ADV

Fixes a bug where goto statement results into network layer skipping
the forwarding of unicast message on the ADV bearer, if the message
gets succesfully sent on the GATT bearer. This is undesirable. Node
has no knowledge of which external entity has which unicast address.
It may be possible that Proxy node can deliberately add unicast
addresses of other nodes to the whitelist to receive some traffic
for sniffing.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>