serial: 8250_exar: Replace msleep(1) with usleep_range()

As explained in Documentation/timers/timers-howto.rst
the small amount of milliseconds sometimes produces
much longer delays.

Replace msleep(1) with usleep_range(1000, 1100).
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190805142535.21948-2-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_exar.c

@@ -180,7 +180,7 @@ static void exar_shutdown(struct uart_port *port)
 			tx_complete = 1;
 		else
 			tx_complete = 0;
-		msleep(1);
+		usleep_range(1000, 1100);
 	} while (!uart_circ_empty(xmit) && !tx_complete && i++ < 1000);
 
 	serial8250_do_shutdown(port);