tty: serial: 8250_mtk: set regshift for mmio32

To use mmio32, we also need to set regshift.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Acked-by: Eddie Huang <eddie.huang@mediatek.com>
Link: https://lore.kernel.org/r/20200915054825.3289105-2-hsinyi@chromium.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>