mt76: mt7615: fix key set/delete issues

mt76: mt7615: fix key set/delete issues

[ Upstream commit 730d6d0da8d8f5905faafe645a5b3c08ac3f5a8f ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>