libata: improve EH report formatting

Requiring LLDs to format multiple error description messages properly
doesn't work too well.  Help LLDs a bit by making ata_ehi_push_desc()
insert ", " on each invocation.  __ata_ehi_push_desc() is the raw
version without the automatic separator.

While at it, make ehi_desc interface proper functions instead of
macros.

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 5d3df6c..94b37d1 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -910,16 +910,9 @@
 /*
  * ata_eh_info helpers
  */
-#define ata_ehi_push_desc(ehi, fmt, args...) do { \
-	(ehi)->desc_len += scnprintf((ehi)->desc + (ehi)->desc_len, \
-				     ATA_EH_DESC_LEN - (ehi)->desc_len, \
-				     fmt , ##args); \
-} while (0)
-
-#define ata_ehi_clear_desc(ehi) do { \
-	(ehi)->desc[0] = '\0'; \
-	(ehi)->desc_len = 0; \
-} while (0)
+extern void __ata_ehi_push_desc(struct ata_eh_info *ehi, const char *fmt, ...);
+extern void ata_ehi_push_desc(struct ata_eh_info *ehi, const char *fmt, ...);
+extern void ata_ehi_clear_desc(struct ata_eh_info *ehi);
 
 static inline void __ata_ehi_hotplugged(struct ata_eh_info *ehi)
 {