net: airoha: Disable GDM2 forwarding before configuring GDM2 loopback

net: airoha: Disable GDM2 forwarding before configuring GDM2 loopback

Hw design requires to disable GDM2 forwarding before configuring GDM2
loopback in airoha_set_gdm2_loopback routine.

Fixes: 9cd451d414f6e ("net: airoha: Add loopback support for GDM2")
Tested-by: Madhur Agrawal <madhur.agrawal@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260520-airoha-disable-gdm2-fwd-v1-1-1eeea5dffc2f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>