greybus: es2: Fix apb_log null pointer exception

Read on apb_log causes null pointer exception due to the missing es2
device pointer passing to the debugfs.

Testing done:
- Enable apb_log and cat it
Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/es2.c

@@ -842,7 +842,7 @@ static void usb_log_enable(struct es2_ap_dev *es2)
 		return;
 	/* XXX We will need to rename this per APB */
 	es2->apb_log_dentry = debugfs_create_file("apb_log", S_IRUGO,
-						gb_debugfs_get(), NULL,
+						gb_debugfs_get(), es2,
 						&apb_log_fops);
 }