net: add description for len argument of dev_get_phys_port_name

When the function dev_get_phys_port_name was added it missed a description
for it's len argument. Adding it.

Fixes: db24a904 ("net: add support for phys_port_name")
Signed-off-by: NLuis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -6445,6 +6445,7 @@ EXPORT_SYMBOL(dev_get_phys_port_id);
  *	dev_get_phys_port_name - Get device physical port name
  *	@dev: device
  *	@name: port name
+ *	@len: limit of bytes to copy to name
  *
  *	Get device physical port name
  */