net: airoha: simplify WAN device check in airoha_dev_init()

net: airoha: simplify WAN device check in airoha_dev_init()

airoha_register_gdm_devices() iterates eth->ports[] in order, so GDM2's
netdev is always registered before GDM3/GDM4. This means the explicit
check for eth->ports[1] && eth->ports[1]->devs[0] is a redundant
special-case of what airoha_get_wan_gdm_dev() already covers, since
GDM2 is always marked as WAN during its own ndo_init.
Remove the redundant check and rely solely on airoha_get_wan_gdm_dev()
which handles both the GDM2-present and GDM2-absent cases.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20260610-airoha-eth-simplify-dev-init-v2-1-8f244e69b0d4@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>