tools/testing/cxl: Remove cxl_test module math loading message

tools/testing/cxl: Remove cxl_test module math loading message

Commit "tools/testing/cxl: Add XOR Math support to cxl_test" added
a module parameter to cxl_test for the interleave_arithmetic option.

In doing so, it also added this dev_dbg() message describing which
option cxl_test used during load:
"[  111.743246] (NULL device *): cxl_test loading modulo math option"
That "(NULL device *)" has raised needless user concern.

Remove the dev_dbg() message and make the module_param readable via
sysfs for users that need to know which math option is active.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/20230126170555.701240-1-alison.schofield@intel.com
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>