spl: mmc: Fix subsequent calls to spl_mmc_load with CONFIG_BLK

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: bf28d9a6 ("spl: mmc: Use correct MMC device when loading image")
Signed-off-by: Sean Anderson <seanga2@gmail.com>