PinePhone: Modem's SMS cache might get full and cannot receive SMS

While I haven't heard much about this issue, except from one person it's still better to report this here.

ModemManager does not delete SMS messages that the phone have received, which builds up from time and might not have any more space to store texts.

Currently the fix is to run AT+CMGD=0,4 on the AT modem, this will delete them and let SMS to come in.