cxl/mbox: Remove bad comment

__cxl_mem_mbox_send_cmd() no longer exists.  Remove the reference.

Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20211102202901.3675568-2-ira.weiny@intel.com


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