net: l2: ppp: Patch carrier lost and L2 enable/disable

L2 PPP is not able to handle the carrier being lost gracefully,
nor is it able to gracefully close the PPP connection when
net_if_down() is called.

This patch refactors the L2 PPP module to use the carrier state
to either properly close or simply terminate the PPP connection.

Additionally, it ensures that the PPP session is closed properly
before calling ppp->stop().

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>