clk: rockchip: Fix static checker warning in rockchip_ddrclk_get_parent call

Fixes the signedness bug returning '(-22)' on the return type by removing the
sanity checker in rockchip_ddrclk_get_parent(). The function should return
and unsigned value only and it's safe to remove the sanity checker as the
core functions that call get_parent like clk_core_get_parent_by_index already
ensures the validity of the clk index returned (index >= core->num_parents).

Fixes: a4f182bf ("clk: rockchip: add new clock-type for the ddrclk")
Cc: stable@vger.kernel.org
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>