net: l2: ppp: Fix terminate requests

When PPP moves to the 'Closing' state it sends up to configured number
of 'Terminate-Request' packets. It only moves to the 'Initial' state,
after running out of retransmits or when it receives a 'Terminate-Ack'
packet.

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>