dm: core: remove unnecessary return condition in driver lookup

The variable "drv" never becomes NULL because ll_entry_start()
always returns a valid pointer even if there are no entries.

The case "n_ents == 0" is covered by the following "for" loop.

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