cmd: nand: remove direct access to struct mtd_info->priv

Replace direct access to struct mtd_info->priv with proper
accessor mtd_to_nand().

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
diff --git a/cmd/nand.c b/cmd/nand.c
index d6542e5..a2152ec 100644
--- a/cmd/nand.c
+++ b/cmd/nand.c
@@ -128,7 +128,7 @@
 	nand_curr_device = dev;
 
 #ifdef CONFIG_SYS_NAND_SELECT_DEVICE
-	board_nand_select_device(mtd->priv, dev);
+	board_nand_select_device(mtd_to_nand(mtd), dev);
 #endif
 
 	return 0;