Bluetooth: hci_serdev: do not open device in hci open

The device driver may need to communicate with the UART
device while the Bluetooth device is closed (e.g. due
to interrupts).

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>