serial: 8520_port: Fix function param documentation

The parameter is named p while the documentation talks about up.
Fix the doc to be in line with the code.

Fixes: 058bc104f7 ("serial: 8250: Generalize rs485 software emulation")
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200517215610.2131618-2-heiko@sntech.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -1432,7 +1432,7 @@ static void serial8250_stop_rx(struct uart_port *port)
 
 /**
  * serial8250_em485_stop_tx() - generic ->rs485_stop_tx() callback
- * @up: uart 8250 port
+ * @p: uart 8250 port
  *
  * Generic callback usable by 8250 uart drivers to stop rs485 transmission.
  */