libata debugging: Warn when unable to find timing descriptor based on xfer_mode

ata_timing_find_mode could return NULL which is not checked by all
low-level ATA drivers using it and cause a NULL ptr deref. Warn at least
so that possible issues can get fixed easily.
Signed-off-by: NBorislav Petkov <bp@alien8.de>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-core.c

@@ -2942,6 +2942,10 @@ const struct ata_timing *ata_timing_find_mode(u8 xfer_mode)
 
 	if (xfer_mode == t->mode)
 		return t;
+
+	WARN_ONCE(true, "%s: unable to find timing for xfer_mode 0x%x\n",
+			__func__, xfer_mode);
+
 	return NULL;
 }