HID: usbhid: paper over wrong bNumDescriptor field

Some faulty devices (ZWO EFWmini) have a wrong optional HID class
descriptor count compared to the provided length.

Given that we plainly ignore those optional descriptor, we can attempt
to fix the provided number so we do not lock out those devices.

Signed-off-by: Benjamin Tissoires <[e-mail]>

drivers/hid/usbhid/hid-core.c

···
 	struct usb_device *dev = interface_to_usbdev (intf);
 	struct hid_descriptor *hdesc;
 	struct hid_class_descriptor *hcdesc;
+	__u8 fixed_opt_descriptors_size;
 	u32 quirks = 0;
 	unsigned int rsize = 0;
 	char *rdesc;
···
 			      (hdesc->bNumDescriptors - 1) * sizeof(*hcdesc)) {
 		dbg_hid("hid descriptor invalid, bLen=%hhu bNum=%hhu\n",
 			hdesc->bLength, hdesc->bNumDescriptors);
-		return -EINVAL;
+
+		/*
+		 * Some devices may expose a wrong number of descriptors compared
+		 * to the provided length.
+		 * However, we ignore the optional hid class descriptors entirely
+		 * so we can safely recompute the proper field.
+		 */
+		if (hdesc->bLength >= sizeof(*hdesc)) {
+			fixed_opt_descriptors_size = hdesc->bLength - sizeof(*hdesc);
+
+			hid_warn(intf, "fixing wrong optional hid class descriptors count\n");
+			hdesc->bNumDescriptors = fixed_opt_descriptors_size / sizeof(*hcdesc) + 1;
+		} else {
+			return -EINVAL;
+		}
 	}
 
 	hid->version = le16_to_cpu(hdesc->bcdHID);