cxl: sparse: Silence iomem warning in debugfs file creation

An IO address, tagged with __iomem, is passed to debugfs_create_file
as private data. This requires that it be cast to void *. The cast
drops the __iomem annotation and so creates a sparse warning:

  drivers/misc/cxl/debugfs.c:51:57: warning: cast removes address space of expression

The address space marker is added back in the file operations
(fops_io_u64).

Silence the warning with __force.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>