mmc: meson-gx: replace cmd->data in meson_mmc_start_cmd

Replace cmd->data with a local variable to simplify code a little.

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