The Quectel EC21 does not provide the SMS PDU on the message event
notification.
This patch adds a call to 'raw read' on the message ID from the event
notification if the event notification does not already contain the
message data.
The message data begins with the SMSC length, type, and address so
the TPDU length is adjusted accordingly in the raw_read callback. This
differs from the way the raw message data is handled in the case
that it is included in the event notification itself. As I don't have
access to any other QMI modem at this time, I'm can not confirm that
this difference is reasonable.
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