net: usb: qmi_wwan: add qmap mux protocol support

This patch adds support for qmap mux protocol available in recent
Qualcomm based modems.

The qmap mux protocol can be used for multiplexing data packets in
order to have multiple ip streams through the same physical device.

Two new sysfs files are added for adding/removing the qmap mux based
interfaces (named qmimux):

- /sys/class/net/<iface>/qmi/add_mux
- /sys/class/net/<iface>/qmi/del_mux

Main patch author is Bjørn Mork <bjorn@mork.no>

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>