spl: blk_fs: Fix uninitialized return value when we can't get a blk_desc

Initialize ret to avoid returning garbage if blk_get_devnum_by_uclass_id
fails.

Fixes: 8ce6a2e1 ("spl: blk: Support loading images from fs")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>