mt76: mt76x0: disable GTK offloading

When operating two VAP on a MT7610 with encryption (PSK2, SAE, OWE),
only the first one to be created will transmit properly encrypteded
frames.

All subsequently created VAPs will sent out frames with the payload left
unencrypted, breaking multicast traffic (ICMP6 NDP) and potentially
disclosing information to a third party.

Disable GTK offloading and encrypt these frames in software to
circumvent this issue. THis only seems to be necessary on MT7610 chips,
as MT7612 is not affected from our testing.

Signed-off-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Felix Fietkau <nbd@nbd.name>