Moving mode attribute to tz->device.groups requires the implementation
of a .is_visible() callback. The condition returned by .is_visible() of
the mode attribute group is kept the same, we allow the attribute to be
visible only if ops->get_mode() is set by the thermal driver.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>