mt76: mt7615: remove key check in mt7615_mcu_set_wtbl_key

Do not check key pointer in mt7615_mcu_set_wtbl_key since if set_key_cmd
is SET_KEY, key will be always not NULL. This patch will address a false
positive reported by Coverity-Scan

Addresses-Coverity-ID: 1445463 ("Dereference after null check")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>