mmc: meson-gx: improve meson_mmc_irq_thread

Remove unneeded variable ret and simplify the if block.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>