mmc: meson-gx: remove unneeded variable in meson_mmc_clk_init

Because the DT requires a fixed number of mux parent clocks,
variable mux_parent_count can be replaced with constant
MUX_CLK_NUM_PARENTS, so remove it.

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>