net: dsa: Fix DSA driver for KSZ8xxx to correctly initialize LAN devices

The commit "drivers: ethernet: dsa_ksz8xxx: use
NET_DEVICE_DT_DEFINE_INSTANCE"
(SHA1: f78a0810) replaced
NET_DEVICE_INIT_INSTANCE() with NET_DEVICE_DT_DEFINE_INSTANCE() to
facilitate the removal of deprecated (from Zephyr's 3.2 release)
DT_LABEL() macro.

Unfortunately, the per LAN port initialization is necessary for correct
operation of the DSA driver - otherwise following errors were visible
when LLDP DSA sample (samples/net/dsa/src) was run on ip_k66f board:

<wrn> net_if: iface 0x20001440 is down
<inf> net_dsa_lldp_sample: LLDP pkt recv -> lan1
<inf> net_dsa_lldp_sample:   CHASSIS ID:     38:05:43:69:XX:ZZ
<inf> net_dsa_lldp_sample:   PORT ID:        38:05:43:69:XX:ZZ
<inf> net_dsa_lldp_sample:   TTL:            120s
<inf> net_dsa_lldp_sample:   SYSTEM NAME:    mtt
<err> net_dsa_lldp_sample: Failed to send, errno 115

The fix is to use again NET_DEVICE_INIT_INSTANCE() with "lan"X name
assigned (to avoid too long names when recommended DT_PROP() is
used instead of DT_LABEL()).

Fixes: f78a0810

Signed-off-by: Lukasz Majewski <lukma@denx.de>