modem: ppp: fix receiving of successive frames when one 0x7E is omitted

The SOF delimiter byte may be omitted when a frame follows another
that just ended with that byte.
The parsing used to expect that second delimiter anyway,
which resulted in PPP frames going missing.

As an additional improvement, dropped bytes as well as the length
of received frames are now (debug) logged.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>