Bluetooth: btusb: Add the missed release_firmware() in btusb_mtk_setup_firmware()

btusb_mtk_setup_firmware() misses to call release_firmware() in an error
path. Jump to err_release_fw to fix it.

Fixes: f6451257 ("Bluetooth: btusb: fix up firmware download sequence")
Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>