dm: core: Correct the return value for uclass_find_first_device()

dm: core: Correct the return value for uclass_find_first_device()

This function returns -ENODEV when there is no device. This is
inconsistent with other functions, such as uclass_find_next_device(),
which returns 0.

Update it and tidy up the incorrect '-1' values in the comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>