net: l2: ppp: ppp uart usage fixed

This fixes 3 issues that came within PR #59124 for ppp uart usage.

Earlier start/stop of ppp was done at enable() but that
was removed in PR #59124. Now putting enable/disable() back and
putting start/stop there.
Additionally, there was a double ppp carrier ON when NET_EVENT_IF_DOWN.
For that net_if_carrier_on/off is set in uart ppp.c driver.
Also, maybe worth to be mentioned that after PR #59124 there is no
ppp carrier off when lcp is disconnected, for workaround that change,
application should use ppp dead/running events.

Signed-off-by: Jani Hirsimäki <jani.hirsimaki@nordicsemi.no>