dm: mmc: Rewrite mmc_blk_probe()

This function is called when the MMC block device is being probed. There
is a recursive call in this function since find_mmc_device() itself can
cause the MMC device to be probed.

Admittedly the MMC device should already be probed, since we would not be
probing its child otherwise, but the current code is unnecessarily
convoluted.

Rewrite this to access the MMC structure directly.

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