spl: mmc: Report device number when we fail

If we fail to find the MMC boot device, report the number of the one
we were looking for in the error to aid diagnosis.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>

common/spl/spl_mmc.c

@@ -140,7 +140,8 @@ static int spl_mmc_find_device(struct mmc **mmcp, u32 boot_device)
 #endif
 	if (err) {
 #ifdef CONFIG_SPL_LIBCOMMON_SUPPORT
-		printf("spl: could not find mmc device. error: %d\n", err);
+		printf("spl: could not find mmc device %d. error: %d\n",
+		       mmc_dev, err);
 #endif
 		return err;
 	}