net: dsa: set name_assign_type to NET_NAME_ENUM for enumerated user ports

When a user port does not have a label in device tree, and we thus
fall back to the eth%d scheme, the proper constant to use is
NET_NAME_ENUM. See also commit e9f656b7 ("net: ethernet: set
default assignment identifier to NET_NAME_ENUM"), which in turn quoted
commit 685343fc

 ("net: add name_assign_type netdev attribute"):

    ... when the kernel has given the interface a name using global
    device enumeration based on order of discovery (ethX, wlanY, etc)
    ... are labelled NET_NAME_ENUM.

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>