wifi: b43: enforce bounds check on firmware key index in b43_rx()

In the Linux kernel, the following vulnerability has been resolved: wifi: b43: enforce bounds check on firmware key index in b43_rx() The firmware-controlled key index in b43_rx() can exceed the dev->key[] array size (58 entries). The existing B43_WARN_ON is non-enforcing in production builds, allowing an out-of-bounds read. Make the B43_WARN_ON check enforcing by dropping the frame when the firmware returns an invalid key index.