mmc: meson-gx: Drop redundant WARN_ON() in the irq handler

The host pointer is already being dereferenced earlier, so let's just drop
the redundant WARN_ON.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20230622105327.77296-1-ulf.hansson@linaro.org