usb: serial: mos7840: Fixup mos7840_chars_in_buffer()

 * Use the buffer content length as opposed to the total buffer size.  This can
   be a real problem when using the mos7840 as a usb serial-console as all
   kernel output is truncated during boot.
Signed-off-by: NMark Ferrell <mferrell@uplogix.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/mos7840.c

@@ -1232,9 +1232,12 @@ static int mos7840_chars_in_buffer(struct tty_struct *tty)
 		return 0;
 
 	spin_lock_irqsave(&mos7840_port->pool_lock, flags);
-	for (i = 0; i < NUM_URBS; ++i)
-		if (mos7840_port->busy[i])
-			chars += URB_TRANSFER_BUFFER_SIZE;
+	for (i = 0; i < NUM_URBS; ++i) {
+		if (mos7840_port->busy[i]) {
+			struct urb *urb = mos7840_port->write_urb_pool[i];
+			chars += urb->transfer_buffer_length;
+		}
+	}
 	spin_unlock_irqrestore(&mos7840_port->pool_lock, flags);
 	dbg("%s - returns %d", __func__, chars);
 	return chars;