mt76: mt7915: fix possible memory leak in mt7915_mcu_add_sta

Free allocated skb in mt7915_mcu_add_sta routine in case of failures.

Fixes: 89bbd373 ("mt76: mt7915: rework starec TLV tags")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>