cxl/mbox: Move raw command warning to raw command validation

cxl/mbox: Move raw command warning to raw command validation

This move serves two purposes: 1) Emit the warning in the raw
command validation path, and 2) Remove the dependency on the
struct cxl_mem_command in handle_mailbox_cmd_from_user() in
preparation for a refactor of that function.

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/df5f0e0ec8afa1f75299aa86b4226ab4479ef325.1648687552.git.alison.schofield@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>