ata: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

In cases where a "drop through" comment was already in place, I replaced
it  with a proper "fall through" comment, which is what GCC is expecting
to find.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/sata_mv.c

@@ -2387,7 +2387,7 @@ static unsigned int mv_qc_issue(struct ata_queued_cmd *qc)
 				      ": attempting PIO w/multiple DRQ: "
 				      "this may fail due to h/w errata\n");
 		}
-		/* drop through */
+		/* fall through */
 	case ATA_PROT_NODATA:
 	case ATAPI_PROT_PIO:
 	case ATAPI_PROT_NODATA:
@@ -3875,7 +3875,7 @@ static int mv_chip_id(struct ata_host *host, unsigned int board_idx)
 				" and avoid the final two gigabytes on"
 				" all RocketRAID BIOS initialized drives.\n");
 		}
-		/* drop through */
+		/* fall through */
 	case chip_6042:
 		hpriv->ops = &mv6xxx_ops;
 		hp_flags |= MV_HP_GEN_IIE;