MMC devices do not have uclass platdata containing blk_descs, only their
child block devices do. Fortunately, we have a function just for this
purpose. This fixes subsequent calls to spl_mmc_load.
Fixes: bf28d9a6599 ("spl: mmc: Use correct MMC device when loading image")
Signed-off-by: Sean Anderson <seanga2@gmail.com>