net: ppp: Properly terminate LCP state at modem side when closing down

Takes the modem state machine by calling lcp_close instead of lcp_down
Using this method the LCP layer sends a TERMINATE_REQ to the modem and the
network interface is only taken down when the LCP layer has properly
finished.
Moved the ppp_mgmt_raise_carrier_off_event and net_if_carrier_down
to lcp.c to avoid breaking the interface.
Tested on a real modem.

Fixes: zephyrproject-rtos#41627

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>