bootstd: Adjust code ordering to work around compiler quirk

bootstd: Adjust code ordering to work around compiler quirk

At present when debugging is off, bootdev_find_in_blk() sometimes fails
to find a valid bootflow, e.g. with virtio. Accessing the 'blk' variable
later in the function seems to correct it.

Move the 'ret' check before the debug statement and set the block device
again aftewards, to work around this.

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