mt76: mt76x0: disable GTK offloading

When the GTK is offloaded, MT7610 won't transmit any multicast frames.
This is most likely due to a bug in the offloading datapath. MT7612 is
not affected.

Disable GTK offloading for now. It can be re-enabled once the bug in the
offloading path is fixed.

Signed-off-by: David Bauer <mail@david-bauer.net>