net: dsa: lantiq_gswip: remove duplicate assignment to vlan_mapping.val[0]

net: dsa: lantiq_gswip: remove duplicate assignment to vlan_mapping.val[0]

When idx == -1 in gswip_vlan_add(), we set vlan_mapping.val[0] = vid,
even though we do the exact same thing again outside the if/else block.

Remove the duplicate assignment.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/039ecb48e038cea856a9a6230ad1543db2bc382d.1760566491.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>