NTB: Add Error Handling in ntb_device_setup

If an error is encountered in ntb_device_setup, it is possible that the
spci_cmd isn't populated.  Writes to the offset can result in a NULL
pointer dereference.  This issue is easily encountered by running in
NTB-RP mode, as it currently is not supported and will generate an
error.  To get around this issue, return if an error is encountered
prior to attempting to write to the spci_cmd offset.

Signed-off-by: Jon Mason <jon.mason@intel.com>

drivers/ntb/ntb_hw.c

@@ -644,10 +644,13 @@ static int ntb_device_setup(struct ntb_device *ndev)
 		rc = -ENODEV;
 	}
 
+	if (rc)
+		return rc;
+
 	/* Enable Bus Master and Memory Space on the secondary side */
 	writew(PCI_COMMAND_MEMORY | PCI_COMMAND_MASTER, ndev->reg_ofs.spci_cmd);
 
-	return rc;
+	return 0;
 }
 
 static void ntb_device_free(struct ntb_device *ndev)