wifi: b43legacy: enforce bounds check on firmware key index in RX path

In the Linux kernel, the following vulnerability has been resolved: wifi: b43legacy: enforce bounds check on firmware key index in RX path Same fix as b43: the firmware-controlled key index in b43legacy_rx() can exceed dev->max_nr_keys. The existing B43legacy_WARN_ON is non-enforcing in production builds, allowing an out-of-bounds read of dev->key[]. Make the check enforcing by dropping the frame for invalid indices.