net: airoha: Fix lan4 support in airoha_qdma_get_gdm_port()

EN7581 SoC supports lan{1,4} ports on MT7530 DSA switch. Fix lan4
reported value in airoha_qdma_get_gdm_port routine.

Fixes: 23020f04 ("net: airoha: Introduce ethernet support for EN7581 SoC")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250304-airoha-eth-fix-lan4-v1-1-832417da4bb5@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>