kernel-device: ID_USB_INTERFACE_NUM should be read as an hex string

The original g_udev_device_get_property_as_int() uses strtol() without
an explicit base (i.e. 0) so that the base is autodetected from the
string whenever possible (e.g. if prefixes with '0x' it is treated as a
hexadecimal string).

But, for ID_USB_INTERFACE_NUM, we explicitly require reading the number
as an hex string, even if we don't have any '0x' prefix.

Reported-by: Matthew Stanger <stangerm2@gmail.com>

plugins/option/mm-plugin-option.c

@@ -56,7 +56,7 @@ grab_port (MMPlugin *self,
 {
     MMPortSerialAtFlag pflags = MM_PORT_SERIAL_AT_FLAG_NONE;
     MMKernelDevice *port;
-    gint usbif;
+    guint usbif;
 
     /* The Option plugin cannot do anything with non-AT ports */
     if (!mm_port_probe_is_at (probe)) {
@@ -73,7 +73,7 @@ grab_port (MMPlugin *self,
      * the modem/data port, per mail with Option engineers.  Only this port
      * will emit responses to dialing commands.
      */
-    usbif = mm_kernel_device_get_property_as_int (port, "ID_USB_INTERFACE_NUM");
+    usbif = mm_kernel_device_get_property_as_int_hex (port, "ID_USB_INTERFACE_NUM");
     if (usbif == 0)
         pflags = MM_PORT_SERIAL_AT_FLAG_PRIMARY | MM_PORT_SERIAL_AT_FLAG_PPP;