cxl/pci: Cleanup cxl_map_device_regs()

Use a loop to reduce the duplicated code in cxl_map_device_regs(). This
is in preparation for deleting cxl_map_regs().

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/166974409867.1608150.14886452053935226038.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>