drm: ARM HDLCD - fix an error code

We accidentally return PTR_ERR(NULL) which is success instead of a
negative error code.

Fixes: 879e40bea6f2 ('drm: ARM HDLCD - get rid of devm_clk_put()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>