port-probe: restart AT probing if "NO CARRIER" is received during QCDM probing

If the system and modem are independently powered, a system restart may leave one of the modem's AT ports in PPP mode. On restart this may cause AT probing to fail. The modem may terminate PPP after receiving bogus HDLC frames from QCDM probing (since PPP also uses HDLC framing), and return "NO CARRIER". We can use this to restart AT probing now that PPP is down and auto-detect the expected additional AT ports.

Feb 08 20:22:53 ModemManager[385]: <debug> [1675887773.887574] [ttyUSB2/at] <-- '\8\158\239~'
Feb 08 20:22:54 ModemManager[385]: <debug> [1675887774.215054] [ttyUSB2/at] <-- '\8\214\137~'
Feb 08 20:22:54 ModemManager[385]: <debug> [1675887774.851349] [ttyUSB2/at] <-- '\8\250\177~'
Feb 08 20:22:55 ModemManager[385]: <debug> [1675887775.576776] [ttyUSB2/at] <-- '\8\168u~'
Feb 08 20:22:55 ModemManager[385]: <debug> [1675887775.612694] [ttyUSB2/at] <-- '\8w\25~'
Feb 08 20:22:55 ModemManager[385]: <debug> [1675887775.862886] [ttyUSB2/at] <-- '\8!&~'
Feb 08 20:22:56 ModemManager[385]: <debug> [1675887776.614747] [ttyUSB2/at] <-- '\8%\254~'
Feb 08 20:22:56 ModemManager[385]: <debug> [1675887776.686207] [ttyUSB2/probe] port is not AT-capable
Feb 08 20:22:56 ModemManager[385]: <debug> [1675887776.686447] [ttyUSB2/probe] probing QCDM...
Feb 08 20:22:56 ModemManager[385]: <debug> [1675887776.725483] [ttyUSB2/qcdm] --> 7e 00 78 f0 7e
Feb 08 20:22:56 ModemManager[385]: <debug> [1675887776.880941] [ttyUSB2/qcdm] <-- 0d 0a 4e 4f 20 43 41 52 52 49 45 52 0d 0a

Signed-off-by: Dan Williams dan@ioncontrol.co