Bluetooth: Cleanup RFCOMM device registration error handling

If RFCOMM tty device registration fails, cleanup by releasing
the tty_port reference to trigger rfcomm_dev destruction
(rather than open-coding it).

The dlc reference release is moved into rfcomm_dev_add(),
which ensures cleanup in both error paths -- ie., if
__rfcomm_dev_add() fails or if tty_port_register_device() fails.

Fixes releasing the module reference if device registration fails.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Tested-By: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>