mt76: mt7921: enable hw offloading for wep keys

Enable wep key hw offloading for sta mode. This patch fixes
WoW support for wep connections.

Tested-by: Deren.Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>