wifi: mt76: mt7996: let upper layer handle MGMT frame protection

The firmware support for management frame protection has limitations:
- do not support cipher BIP-GMAC-128 and BIP-GMAC-256
- support cipher BIP-CMAC-128 and BIP-CMAC-256, except action frame with
  action type 'not robust'.

Therefore, to simplify the logic, do not set the IGTK to firmware and
let the encryption of management frames 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>