Bluetooth: hci_ll: 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().

Fixes: 166d2f6a

 ("[Bluetooth] Add UART driver for Texas Instruments' BRF63xx chips")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>