mmc: meson-gx: ack only raised irq

This is merely a clean up. It makes sense to only ack raised irqs
instead of acking everything all the time.

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