dm: do not check the existence of uclass operation

The function uclass_add() checks uc_drv->ops as follows:

        if (uc_drv->ops) {
                dm_warn("No ops for uclass id %d\n", id);
                return -EINVAL;
        }

It seems odd because it warns "No ops" when uc_drv->ops has
non-NULL pointer.  (Looks opposite.)

Anyway, most of UCLASS_DRIVER entries have no .ops member.
This check makes no sense.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>