modem.py: accept hex chars in ICCID (#38735)

E.118 specifies decimal digits, but many real SIMs carry hex characters
in EF_ICCID (e.g. China Mobile's 898600B5... range, some MVNO/IoT SIMs).
AT+QCCID returns them verbatim, and the strict isdigit() check blanked
the ICCID, leaving the modem daemon stuck in INITIALIZING forever and
cellular dead. ModemManager parses ICCID as hex for the same reason.

Verified on a comma four with a China Mobile SIM (EG916Q-GL): previously
stuck retrying 'identity read incomplete', now dials and passes traffic.

(cherry picked from commit e10c0fd960016e385f1135d9af4af03b634a7ad2)

openpilot/common/hardware/comma/modem.py

@@ -5,6 +5,7 @@ import logging
 import os
 import select
 import signal
+import string
 import struct
 import subprocess
 import tempfile
@@ -354,7 +355,7 @@ class Modem:
       imei = ""
 
     iccid = (self._atv("AT+QCCID", "+QCCID:") or "").rstrip("F")
-    if not iccid.isdigit():
+    if not all(c in string.hexdigits for c in iccid):
       iccid = ""
 
     imsi = first_line("AT+CIMI")