dm: spl: mmc: Fix EXT SPL support

The patch
"dm: spl: mmc: Support CONFIG_BLK in SPL MMC"
(sha1: 87bce4e5)
converted FAT part of spl_mmc_do_fs_boot() but forget to update also EXT
part by 's/&mmc->block_dev/mmc_get_blk_desc(mmc)/'.
This patch is fixing compilation error when CONFIG_SPL_EXT_SUPPORT
is enabled.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>