serial: 8250_core: actually limit char reads to max_count

In serial8250_rx_chars(), max_count is set to 256. Due to the
post-decrement operator used in the while() condition, the maximum
number of iterations actually 257. This is not a problem, but it is
mildly surprising if you're debugging. Use pre-decrement instead.
Signed-off-by: Frans Klaver <frans.klaver@xsens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_core.c

@@ -1501,7 +1501,7 @@ serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)
 
 ignore_char:
 		lsr = serial_in(up, UART_LSR);
-	} while ((lsr & (UART_LSR_DR | UART_LSR_BI)) && (max_count-- > 0));
+	} while ((lsr & (UART_LSR_DR | UART_LSR_BI)) && (--max_count > 0));
 	spin_unlock(&port->lock);
 	tty_flip_buffer_push(&port->state->port);
 	spin_lock(&port->lock);