tty: serial: 8250_mtk: Fix quot calculation

The calculation of value quot for highspeed register set to three
was wrong. This patch fixes the calculation so that the serial port
for baudrates bigger then 576000 baud is working correctly.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>