Bluetooth: btusb: Handle download_firmware failure cases

For Mediatek chipset, it can not enabled if there are something wrong
in btmtk_setup_firmware_79xx(). Thus, the process must be terminated
and returned error code.

Fixes: fc342c4d ("Bluetooth: btusb: Add protocol support for MediaTek MT7921U USB devices")
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Mark Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>