mmc: meson-gx: simplify bounce buffer setting in meson_mmc_start_cmd

Core ensures that there are no commands with cmd->data being set and
nothing to transfer. And we don't have to reset bit CMD_CFG_DATA_NUM
because cmd_cfg was zero-initialized and this bit isn't set.

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>