Bluetooth: hci_uart: Restore hci_dev->flush callback on open()

For reasons explained in detail in commit 3611f4d2 ("hci_ldisc:
fix null pointer deref") the hci_uart_close() functions sets
hci_dev->flush to NULL.

But the device may be re-opened after a close, this commit restores the
hci_dev->flush callback on open().

Note this commit also moves the nearly empty defition of hci_uart_open()
a bit down in the file to avoid the need for forward declaring
hci_uart_flush().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>