thermal: rockchip: fix an error code

There is a copy and paste bug, "->clk" vs "->pclk", so we return the
wrong error code here.

Fixes: cbac8f63 ('thermal: rockchip: add driver for thermal')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>