qmi_wwan: avoid RCU stalls on device disconnect when in QMAP mode

Switch qmimux_unregister_device() and qmi_wwan_disconnect() to
use unregister_netdevice_queue() and unregister_netdevice_many()
instead of unregister_netdevice(). This avoids RCU stalls which
have been observed on device disconnect in certain setups otherwise.

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>