Bluetooth: hci_ldisc: Ensure hu->hdev set to NULL before freeing hdev

When hci_register_dev() fails, hu->hdev should be set to NULL before
freeing hdev. This avoids potential use of hu->hdev after it has been
freed.

This commit sets hu->hdev to NULL before calling hci_free_dev() in error
handling scenarios in hci_uart_init_work() and hci_uart_register_dev().

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>