bootstd: Probe the block device before use

bootstd: Probe the block device before use

In some cases the block device is obtained but is not probed, since it
is a sibling of the bootdev. Make sure it is probed, so it can be used
without any trouble.

This fixes a bug with virtio, where the device is accessed before it has
been set up by the virtio uclass.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 4d0bd6d0512 ("bootstd: Add the bootdev uclass")
Reported-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>