staging: vt6656: Fix non zero logical return of, usb_control_msg

Starting with commit 59608cb1
("staging: vt6656: clean function's error path in usbpipe.c")
the usb control functions have returned errors throughout driver
with only logical variable checking.

However, usb_control_msg return the amount of bytes transferred
this means that normal operation causes errors.

Correct the return function so only return zero when transfer
is successful.

Cc: stable <stable@vger.kernel.org> # v5.3+
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/08e88842-6f78-a2e3-a7a0-139fec960b2b@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6656/usbpipe.c

@@ -59,7 +59,9 @@ int vnt_control_out(struct vnt_private *priv, u8 request, u16 value,
 
 	kfree(usb_buffer);
 
-	if (ret >= 0 && ret < (int)length)
+	if (ret == (int)length)
+		ret = 0;
+	else
 		ret = -EIO;
 
 end_unlock:
@@ -103,7 +105,9 @@ int vnt_control_in(struct vnt_private *priv, u8 request, u16 value,
 
 	kfree(usb_buffer);
 
-	if (ret >= 0 && ret < (int)length)
+	if (ret == (int)length)
+		ret = 0;
+	else
 		ret = -EIO;
 
 end_unlock: