mt76: mt7615: Fix memory leak in mt7615_mcu_wtbl_sta_add()

mt76: mt7615: Fix memory leak in mt7615_mcu_wtbl_sta_add()

In mt7615_mcu_wtbl_sta_add(), an skb sskb is allocated. If the
subsequent call to mt76_connac_mcu_alloc_wtbl_req() fails, the function
returns an error without freeing sskb, leading to a memory leak.

Fix this by calling dev_kfree_skb() on sskb in the error handling path
to ensure it is properly released.

Fixes: 99c457d902cf9 ("mt76: mt7615: move mt7615_mcu_set_bmc to mt7615_mcu_ops")
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20251113062415.103611-1-zilin@seu.edu.cn
Signed-off-by: Felix Fietkau <nbd@nbd.name>