net: airoha: Fix loopback mode configuration for GDM2 port

Add missing configuration for loopback mode in airhoha_set_gdm2_loopback
routine.

Fixes: 9cd451d4 ("net: airoha: Add loopback support for GDM2")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20251008-airoha-loopback-mode-fix-v2-1-045694fe7f60@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>