net: dsa: mv88e6xxx: Provide dummy implementations for trunk setters

Support for Global 2 registers is build-time optional. In the case
where it was not enabled the build would fail as no "dummy"
implementation of these functions was available.

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