spl: mmc: Fix compiler warning with CONFIG_DM_MMC

spl: mmc: Fix compiler warning with CONFIG_DM_MMC

Since commit 4188ba3 we get the following warning on rockchip boards:

common/spl/spl_mmc.c:31:24: warning: ‘mmc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  count = mmc->block_dev.block_read(0, sector, 1, header);
                        ^
common/spl/spl_mmc.c:251:14: note: ‘mmc’ was declared here
  struct mmc *mmc;

Correct this by move the variable init earlier.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>