Bluetooth: Refactor rfcomm_dev_add()

Move rfcomm_dev allocation and initialization into new function,
__rfcomm_dev_add(), to simplify resource release in error handling.

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