net: dsa: refactor name assignment for user ports

The following two patches each have a (small) chance of causing
regressions for userspace and will in that case of course need to be
reverted.

In order to prepare for that and make those two patches independent
and individually revertable, refactor the code which sets the names
for user ports by moving the "fall back to eth%d if no label is given
in device tree" to dsa_slave_create().

No functional change (at least none intended).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.faineli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>