mmc: meson-gx: remove member parent_mux from struct meson_host

Member mux_parent isn't used outside meson_mmc_clk_init. So remove it
and replace it with a local variable in meson_mmc_clk_init.

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>