mtd: rawnand: Annotate implicit fall through in nand_command/nand_command_lp

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1).

This commit removes the following warnings:

  drivers/mtd/nand/raw/nand_legacy.c:332:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
  drivers/mtd/nand/raw/nand_legacy.c:483:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

drivers/mtd/nand/raw/nand_legacy.c

@@ -331,6 +331,7 @@ static void nand_command(struct nand_chip *chip, unsigned int command,
 		 */
 		if (column == -1 && page_addr == -1)
 			return;
+		/* fall through */
 
 	default:
 		/*
@@ -483,7 +484,7 @@ static void nand_command_lp(struct nand_chip *chip, unsigned int command,
 		chip->legacy.cmd_ctrl(chip, NAND_CMD_NONE,
 				      NAND_NCE | NAND_CTRL_CHANGE);
 
-		/* This applies to read commands */
+		/* fall through - This applies to read commands */
 	default:
 		/*
 		 * If we don't have access to the busy pin, we apply the given