USB: serial: digi_acceleport: reduce chars_in_buffer over-reporting

Due to an ancient quirk in n_tty poll implementation, the
digi_acceleport driver has been reporting that its queue contains 256
(WAKEUP_CHARS) characters whenever its write URB is in use.

This has not been necessary since 2003 when the line-discipline started
taking the write room into account so let's return the maximum transfer
size again in order to over-report a little less and incidentally fix
the related debug statement.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/digi_acceleport.c

@@ -1048,8 +1048,7 @@ static unsigned int digi_chars_in_buffer(struct tty_struct *tty)
 	if (priv->dp_write_urb_in_use) {
 		dev_dbg(&port->dev, "digi_chars_in_buffer: port=%d, chars=%d\n",
 			priv->dp_port_num, port->bulk_out_size - 2);
-		/* return(port->bulk_out_size - 2); */
-		return 256;
+		return port->bulk_out_size - 2;
 	} else {
 		dev_dbg(&port->dev, "digi_chars_in_buffer: port=%d, chars=%d\n",
 			priv->dp_port_num, priv->dp_out_buf_len);