qmi_wwan: add network device usage statistics for qmimux devices

Add proper network device usage statistics for qmimux devices
instead of reporting all-zero values for them.

Fixes: c6adf779 ("net: usb: qmi_wwan: add qmap mux protocol support")
Cc: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: Reinhard Speyerer <rspmn@arcor.de>
Signed-off-by: David S. Miller <davem@davemloft.net>