Bluetooth: hci_ll: Fix download_firmware() return when __hci_cmd_sync fails

When __hci_cmd_sync() fails, download_firmware() should also fail, and
the same error value should be returned as PTR_ERR(skb).

Without this fix, download_firmware() will return a success when it actually
failed in __hci_cmd_sync().

Fixes: 37180552 ("bluetooth: hci_uart: add LL protocol serdev driver support")
Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>