mmc: meson-gx: remove member mrq from struct meson_host

Struct mmc_command includes a reference to the related mmc_request.
Therefore we don't have to store mrq separately in struct meson_host.
And we can remove some now unneeded WARN_ON's.

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