tty: serial: Fix mediatek UART driver setting baudrate issue

In mtk8250_set_termios function, calculating quot value can not be zero,
otherwise, using DIV_ROUND_CLOSEST(port->uartclk, quot * baud) will fail due to
divisor is zero.

Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>