Bluetooth: btusb: Do not call kfree_skb() under spin_lock_irqsave()

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

Fixes: baac6276 ("Bluetooth: btusb: handle mSBC audio over USB Endpoints")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>