net: ppp: Fix PPP connection with host

A follow up to commit 1d7a077e - apparently the PPP interface should
not be brought entirely down internally, as this can break further
communication with the host. Because of that, reintroduce functionality
that used to be covered by net_if_carrier_down() (which basically
skips the L2 enable(false) call), but limited to PPP scope only.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>