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

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.

Fixes: 75388acd0cd8 ("[B43LEGACY]: add mac80211-based driver for legacy BCM43xx devices")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Link: https://patch.msgid.link/20260417111145.2694196-2-tristmd@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>