wifi: mt76: mt7996: mark GCMP IGTK unsupported

Since the FW does not support to handle the integrity and validation of
IGTK in GCMP mode, return -EOPNOTSUPP to let it be handled by upper layer.

Signed-off-by: Michael-CY Lee <michael-cy.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>