mmc: meson-gx: correct irq flag

There is no reason for another device to request the MMC irq. It should
only be used the MMC device, so remove IRQ_SHARED and replace by
IRQ_ONESHOT as we don't the irq to fire again until the irq thread is
done

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>