mt76: mt7921: fix key set/delete issue

Similar to the mt7915 driver, deleting a key with the previous key index
deletes the current key. Rework the code to better keep track of
multiple keys and check for the key index before deleting the current
key

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>