libata: clean up EH speed down implementation

Clean up EH speed down implementation.

* is_io boolean variable is replaced eflags.  is_io is ATA_EFLAG_IS_IO.

* Error categories now have names.

* Better comments.

* Reorder 5min and 10min rules in ata_eh_speed_down_verdict()

* Use local variable @link to cache @dev->link in ata_eh_speed_down()

These changes are to improve readability and ease further changes.
This patch doesn't introduce any behavior change.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/include/linux/libata.h b/include/linux/libata.h
index ca347b0..74f1255 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -482,7 +482,7 @@
 };
 
 struct ata_ering_entry {
-	int			is_io;
+	unsigned int		eflags;
 	unsigned int		err_mask;
 	u64			timestamp;
 };