pci: Fix device_find_first_child() return value handling

This function only ever returns 0, but may not assign the second
parameter. Same thing for device_find_next_child(). Do not assign
ret to stop proliferation of this misuse.

Reported-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>