Bluetooth: btusb: fix non-atomic allocation in completion handler

USB completion handlers are called in atomic context and must
specifically not allocate memory using GFP_KERNEL.

Fixes: a1c49c43 ("Bluetooth: btusb: Add protocol support for MediaTek MT7668U USB devices")
Cc: stable <stable@vger.kernel.org>     # 5.3
Cc: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>