qmi_wwan: extend permitted QMAP mux_id value range

Permit mux_id values up to 254 to be used in qmimux_register_device()
for compatibility with ip(8) and the rmnet driver.

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>