tty/serial: at91: BUG: disable interrupts when !UART_ENABLE_MS()

In set_termios(), interrupts where not disabled if UART_ENABLE_MS() was
false.

Tested on at91sam9g35.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Cc: stable <stable@vger.kernel.org> # >= 3.16
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>