net: dsa: make "label" property optional for dsa2

In the new DTS bindings for DSA (dsa2), the "ethernet" and "link"
phandles are respectively mandatory and exclusive to CPU port and DSA
link device tree nodes.

Simplify dsa2.c a bit by checking the presence of such phandle instead
of checking the redundant "label" property.

Then the Linux philosophy for Ethernet switch ports is to expose them to
userspace as standard NICs by default. Thus use the standard enumerated
"eth%d" device name if no "label" property is provided for a user port.
This allows to save DTS files from subjective net device names.

If one wants to rename an interface, udev rules can be used as usual.

Of course the current behavior is unchanged, and the optional "label"
property for user ports has precedence over the enumerated name.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: David S. Miller <davem@davemloft.net>