clk: rockchip: fix typo in rk3188 spdif_frac parent

Fix typo in common_clk_branches.
Make spdif_pre parent of spdif_frac.

Fixes: 66746420 ("clk: rockchip: include downstream muxes into fractional dividers")
Cc: stable@vger.kernel.org
Signed-off-by: Johan Jonker <jbx9999@hotmail.com>
Acked-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>