HID: wacom: generic: only switch the mode on devices with LEDs

Currently, the driver will attempt to set the mode on all
devices with a center button, but some devices with a center
button lack LEDs, and attempting to set the LEDs on devices
without LEDs results in the kernel error message of the form:

"leds input8::wacom-0.1: Setting an LED's brightness failed (-32)"

This is because the generic codepath erroneously assumes that the
BUTTON_CENTER usage indicates that the device has LEDs, the
previously ignored TOUCH_RING_SETTING usage is a more accurate
indication of the existence of LEDs on the device.

Fixes: 10c55cac

 ("HID: wacom: generic: support LEDs")
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>