net: airoha: Rely on airoha_gdm_dev pointer in airoha_is_lan_gdm_port()

net: airoha: Rely on airoha_gdm_dev pointer in airoha_is_lan_gdm_port()

Rename airoha_is_lan_gdm_port in airoha_is_lan_gdm_dev. Moreover, rely
on airoha_gdm_dev pointer in airoha_is_lan_gdm_dev() instead of
airoha_gdm_port one.
This is a preliminary patch to support multiple net_devices connected to
the same GDM{3,4} port via an external hw arbiter.

Tested-by: Xuegang Lu <xuegang.lu@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260527-airoha-eth-multi-serdes-preliminary-v1-3-ec6ed73ef7fc@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>