port-qmi: allow setting initial mux-id value when using rmnet multiplexing

Qualcomm suggests to use mux id values >= 0x81: though this does not
seem a strict mandatory rule, there are scenarios in which the usage of
a specifix mux id is required otherwise various kind of issues could
happen (e.g. call failure, limited throughput...).

A new ID_MM_INITIAL_QMAP_MUX_ID udev tag allows setting the first initial
mux id value to be used when creating the multiplexed data connection.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>

include/ModemManager-tags.h

@@ -323,6 +323,20 @@
  */
 #define ID_MM_TTY_AT_PROBE_TRIES "ID_MM_TTY_AT_PROBE_TRIES"
 
+/**
+ * ID_MM_INITIAL_QMAP_MUX_ID:
+ *
+ * This is a device-specific tag that allows users to specify the initial value
+ * of the QMAP multiplexed id to be used.
+ *
+ * An integer value greater or equal than 1 and smaller than 255.
+ *
+ * This setting does nothing if the modem doesn't support multiplexing.
+ *
+ * Since: 1.26
+ */
+#define ID_MM_INITIAL_QMAP_MUX_ID "ID_MM_INITIAL_QMAP_MUX_ID"
+
 /*
  * The following symbols are deprecated. We don't add them to -compat
  * because this -tags file is not really part of the installed API.