Bluetooth: hci_uart: add missing NULL check in h5_enqueue

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: hci_uart: add missing NULL check in h5_enqueue Syzbot hit general protection fault in __pm_runtime_resume(). The problem was in missing NULL check. hu->serdev can be NULL and we should not blindly pass &serdev->dev somewhere, since it will cause GPF.