libata: straighten out ATA_ID_* constants

* Kill _OFS suffixes in ATA_ID_{SERNO|FW_REV|PROD}_OFS for consistency
  with other ATA_ID_* constants.

* Kill ATA_SERNO_LEN

* Add and use ATA_ID_SERNO_LEN, ATA_ID_FW_REV_LEN and ATA_ID_PROD_LEN.
  This change also makes ata_device_blacklisted() use proper length
  for fwrev.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/drivers/ata/sata_sil.c b/drivers/ata/sata_sil.c
index 7808d03..d27ab31 100644
--- a/drivers/ata/sata_sil.c
+++ b/drivers/ata/sata_sil.c
@@ -541,9 +541,9 @@
 {
 	int print_info = ap->eh_context.i.flags & ATA_EHI_PRINTINFO;
 	unsigned int n, quirks = 0;
-	unsigned char model_num[41];
+	unsigned char model_num[ATA_ID_PROD_LEN + 1];
 
-	ata_id_c_string(dev->id, model_num, ATA_ID_PROD_OFS, sizeof(model_num));
+	ata_id_c_string(dev->id, model_num, ATA_ID_PROD, sizeof(model_num));
 
 	for (n = 0; sil_blacklist[n].product; n++)
 		if (!strcmp(sil_blacklist[n].product, model_num)) {