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: c6adf77953 ("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>

Documentation/ABI/testing/sysfs-class-net-qmi

@@ -29,7 +29,7 @@ Contact:	Bjørn Mork <bjorn@mork.no>
 Description:
 		Unsigned integer.
 
-		Write a number ranging from 1 to 127 to add a qmap mux
+		Write a number ranging from 1 to 254 to add a qmap mux
 		based network device, supported by recent Qualcomm based
 		modems.
 
@@ -46,5 +46,5 @@ Contact:	Bjørn Mork <bjorn@mork.no>
 Description:
 		Unsigned integer.
 
-		Write a number ranging from 1 to 127 to delete a previously
+		Write a number ranging from 1 to 254 to delete a previously
 		created qmap mux based network device.

drivers/net/usb/qmi_wwan.c

@@ -429,8 +429,8 @@ static ssize_t add_mux_store(struct device *d,  struct device_attribute *attr, c
 	if (kstrtou8(buf, 0, &mux_id))
 		return -EINVAL;
 
-	/* mux_id [1 - 0x7f] range empirically found */
-	if (mux_id < 1 || mux_id > 0x7f)
+	/* mux_id [1 - 254] for compatibility with ip(8) and the rmnet driver */
+	if (mux_id < 1 || mux_id > 254)
 		return -EINVAL;
 
 	if (!rtnl_trylock())