net: l2: ppp: Terminate connection with dormant

When PPP net_if is changed to dormant, the PPP connection
is terminated asynchronously. This is used so that the driver
can terminate the PPP connection gracefully.

Note: net_if_down() is not used as it would require synchronous
operation which would block the system workqueue thus causing
deadlock.

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