net: dsa: mv88e6xxx: Only allow LAG offload on supported hardware

There are chips that do have Global 2 registers, and therefore trunk
mapping/mask tables are not available. Refuse the offload as early as
possible on those devices.

Fixes: 57e661aa ("net: dsa: mv88e6xxx: Link aggregation support")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>