mmc: meson-gx: fix mux mask definition

CCF generic mux will shift the mask using the value defined in shift
Define the mask accordingly

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>