net: usb: qmi_wwan: allow qmimux add/del with master up

There's no reason for preventing the creation and removal
of qmimux network interfaces when the underlying interface
is up.

This makes qmi_wwan mux implementation more similar to the
rmnet one, simplifying userspace management of the same
logical interfaces.

Fixes: c6adf779

 ("net: usb: qmi_wwan: add qmap mux protocol support")
Reported-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>