mmc: meson-gx: improve variable usage in meson_mmc_start_cmd

Simplify the code by using two variables instead of a struct sd_emmc_desc.

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