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: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>