mtd: cfi: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple break statements and a return
instead of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Acked-by: Vignesh Raghavendra <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/20210305081933.GA137147@embeddedor

drivers/mtd/chips/cfi_cmdset_0001.c

···
 			   anyway? The latter for now. */
 			printk(KERN_NOTICE "Flash device refused suspend due to active operation (state %d)\n", chip->state);
 			ret = -EAGAIN;
+			break;
 		case FL_PM_SUSPENDED:
 			break;
 		}

drivers/mtd/chips/cfi_cmdset_0002.c

···
 			/* Someone else might have been playing with it. */
 			goto retry;
 		}
+		return 0;
 
 	case FL_READY:
 	case FL_CFI_QUERY:
···
 			 * as the whole point is that nobody can do anything
 			 * with the chip now anyway.
 			 */
+			break;
 		case FL_PM_SUSPENDED:
 			break;
 

drivers/mtd/chips/cfi_cmdset_0020.c

···
 			 * as the whole point is that nobody can do anything
 			 * with the chip now anyway.
 			 */
+			break;
+
 		case FL_PM_SUSPENDED:
 			break;