cxl/pci: Use module_pci_driver

Now that cxl_mem_{init,exit} no longer need to manage debugfs, switch
back to the smaller form of the boiler plate.

Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116435825.2460985.7201322215431441130.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>