PTP: convert class code to use dev_groups

The dev_attrs field of struct class is going away soon, dev_groups
should be used instead.  This converts the ptp class code to use the
correct field.

Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
index 69d3207..df03f2e 100644
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h
@@ -84,7 +84,7 @@
  * see ptp_sysfs.c
  */
 
-extern struct device_attribute ptp_dev_attrs[];
+extern const struct attribute_group *ptp_groups[];
 
 int ptp_cleanup_sysfs(struct ptp_clock *ptp);