The slave MII bus registered by the DSA code is using the parent MII bus
as part of its name (ds->master_mii_bus_id), in case the parent MII bus
name is already 16 characters long (such as d0072004.mdio-mi) we will
get the following WARN_ON in dsa_switch_setup() when calling
mdiobus_register():

[   79.088782] ------------[ cut here ]------------
[   79.093448] WARNING: at fs/sysfs/dir.c:536 sysfs_add_one+0x80/0xa0()
[   79.099831] sysfs: cannot create duplicate filename
'/class/mdio_bus/d0072004.mdio-mi'

This is a genuine warning, because the DSA slave MII bus will also be
named d0072004.mdio-mi, and since MII_BUS_ID_SIZE is 17 characters long
(with null-terminator) the following will truncate the slave MII bus id:

snprintf(ds->slave_mii_bus->id, MII_BUS_ID_SIZE, "%s-%d:%.2x",
                        ds->master_mii_bus->id, ds->pd->sw_addr);

Fix this by using dsa-<switch index->:<sw_add> which is guaranteed to be
unique.
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>