Bluetooth: btrtl: Fix an issue for the incorrect error return code.

It does not need the '-' for PTR_ERR(skb) because PTR_ERR(skb) will
return the negative value during errors.

Signed-off-by: Max Chou <max.chou@realtek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>