wifi: libertas: if_usb: don't call kfree_skb() under spin_lock_irqsave()

It is not allowed to call kfree_skb() from hardware interrupt
context or with interrupts being disabled. So replace kfree_skb()
with dev_kfree_skb_irq() under spin_lock_irqsave(). Compile
tested only.

Fixes: a3128fee ("libertas: use irqsave() in USB's complete callback")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221207150008.111743-3-yangyingliang@huawei.com