mmc: meson-gx: adjust and re-use constant IRQ_EN_MASK

Constant IRQ_EN_MASK has no user currently. In preparation of adding
SDIO interrupt support, revive it and adjust it to our needs.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/8056622f-2adf-4763-7423-9ccdf4ca78e1@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>