net: usb: qmi_wwan: add qmap id sysfs file for qmimux interfaces

Add qmimux interface sysfs file qmap/mux_id to show qmap id set
during the interface creation, in order to provide a method for
userspace to associate QMI control channels to network interfaces.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>