dm: core: remove unnecessary return condition in uclass lookup

These conditions never happen.
 - There is no real uclass with UCLASS_INVALID id.
 - uclass never becomes NULL because ll_entry_start() always returns
   a valid pointer.

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