V4L/DVB: uvcvideo: Make button controls work properly

According to the v4l2 spec, writing any value to a button control should
result in the action belonging to the button control being triggered.
UVC cams however want to see a 1 written, this patch fixes this by
overriding whatever value user space passed in with -1 (0xffffffff) when
the control is a button control.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/uvc/uvc_ctrl.c

@@ -698,6 +698,14 @@ static void uvc_set_le_value(struct uvc_control_mapping *mapping,
 	int offset = mapping->offset;
 	__u8 mask;
 
+	/* According to the v4l2 spec, writing any value to a button control
+	 * should result in the action belonging to the button control being
+	 * triggered. UVC devices however want to see a 1 written -> override
+	 * value.
+	 */
+	if (mapping->v4l2_type == V4L2_CTRL_TYPE_BUTTON)
+		value = -1;
+
 	data += offset / 8;
 	offset &= 7;