cxl/pci: Use CXL_MBOX_SUCCESS to check against mbox_cmd return code

Also mention the need for the caller to check against any
errors from the hardware in return_code.
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed by: Adam Manzanares <a.manzanares@samsung.com>
Link: https://lore.kernel.org/r/20220404021216.66841-3-dave@stgolabs.netSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/pci.c

@@ -177,9 +177,9 @@ static int __cxl_pci_mbox_send_cmd(struct cxl_dev_state *cxlds,
 	mbox_cmd->return_code =
 		FIELD_GET(CXLDEV_MBOX_STATUS_RET_CODE_MASK, status_reg);
 
-	if (mbox_cmd->return_code != 0) {
+	if (mbox_cmd->return_code != CXL_MBOX_SUCCESS) {
 		dev_dbg(dev, "Mailbox operation had an error\n");
-		return 0;
+		return 0; /* completed but caller must check return_code */
 	}
 
 	/* #7 */