spl: mmc: Fix compiler warning with CONFIG_DM_MMC

Since commit 4188ba32 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: NSimon Glass <sjg@chromium.org>
Tested-by: NMichal Simek <michal.simek@xilinx.com>
Acked-by: NNikita Kiryanov <nikita@compulab.co.il>

common/spl/spl_mmc.c

@@ -248,7 +248,7 @@ int spl_mmc_do_fs_boot(struct mmc *mmc)
 
 int spl_mmc_load_image(u32 boot_device)
 {
-	struct mmc *mmc;
+	struct mmc *mmc = NULL;
 	u32 boot_mode;
 	int err = 0;
 	__maybe_unused int part;