clk: rockchip: don't return NULL when failing to register ddrclk branch

rockchip_clk_register_ddrclk should not return NULL when failing
to call clk_register, otherwise rockchip_clk_register_branches
prints "unknown clock type". The actual case is that it's a known
clock type but we fail to register it, which may makes user confuse
the reason of failure. And the pr_err here is pointless as
rockchip_clk_register_branches will also print the similar message.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>