switchtec: Use new cdev_device_add() helper function

Convert from "cdev_add() + device_add()" to cdev_device_add(), and from
"device_del() + cdev_del()" to cdev_device_del().

[bhelgaas: changelog]
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>