rapidio: utilize new cdev_device_add helper function

This driver did not originally set kobj.parent so it likely had
potential a use after free bug which this patch fixes.

We convert from device_register to device_initialize/cdev_device_add.
While we are at it we use put_device instead of kfree (as recommended
by the device_initialize documentation). We also remove an unnecessary
extra get_device from the code.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>