HID: multitouch: force retrieving of Win8 signature blob

The Synaptics 0x11e5 over I2C found in the Asus T100-CHI requires to
fetch the signature blob to actually start sending events.

With this patch, we should be close enough to the Windows driver which
checks the content of the blob at plugin to validate or not the
touchscreen.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=113481
Fixes: 6d4f5440 ("HID: multitouch: Fetch feature reports on demand for Win8 devices")
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-multitouch.c

@@ -396,6 +396,11 @@ static void mt_feature_mapping(struct hid_device *hdev,
 			td->is_buttonpad = true;
 
 		break;
+	case 0xff0000c5:
+		/* Retrieve the Win8 blob once to enable some devices */
+		if (usage->usage_index == 0)
+			mt_get_feature(hdev, field->report);
+		break;
 	}
 }