clk: rockchip: Remove redundant null check before clk_prepare_enable

Because clk_prepare_enable() already checked NULL clock parameter,
so the additional check is unnecessary, just remove it.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20201127090551.50254-1-vulab@iscas.ac.cn


Signed-off-by: Heiko Stuebner <heiko@sntech.de>