mt76: mt7615: fix key set/delete issues

There were multiple issues in the current key set/remove code:
- deleting a key with the previous key index deletes the current key
- BIP key would only be uploaded correctly initially and corrupted on rekey

Rework the code to better keep track of multiple keys and check for the
key index before deleting the current key

Signed-off-by: Felix Fietkau <nbd@nbd.name>