cxl/mbox: Remove dependency on cxl_mem_command for a debug msg

In preparation for removing access to struct cxl_mem_command,
change this debug message to use cxl_mbox_cmd fields instead.
Retrieve the pretty command name from cxl_mbox_cmd using a new
opcode to command name helper.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/57265751d336a6e95f5ca31a9c77189408b05742.1648687552.git.alison.schofield@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>