drm/rockchip: cdn-dp: return error code when clk_get_rate failed

The clk_get_rate return 0 if something goes wrong, so it can never be
less then zero, the ret should be set a error code, otherwise the
cdn_dp_clk_enable will return 0 when it failed at clk_get_rate.
In addition, clk_get_rate() returns an "unsigned long", so use
"unsigned long" instead of "u32" is better.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1488940077-22297-2-git-send-email-zyw@rock-chips.com