wifi: libertas: main: 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: d2e7b342 ("libertas: disable functionality when interface is down")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221207150008.111743-4-yangyingliang@huawei.com