mmc: meson-gx: simplify setting timeout configuration parameters

We don't need variable cmd_cfg_timeout, so remove it and simplify the
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>