dm: blk: Tidy up obtaining a block device from its parent

This function now finds its block-device child by looking for a child
device of the correct uclass (UCLASS_BLK). It cannot produce a device of
any other type, so drop the superfluous check.

Provide a version which does not probe the device, since that is often
needed when setting up the device's platdata.

Also fix up the function's comment.

Signed-off-by: Simon Glass <sjg@chromium.org>