HID: wacom: leds: Don't try to control the EKR's read-only LEDs

HID: wacom: leds: Don't try to control the EKR's read-only LEDs

BugLink: http://bugs.launchpad.net/bugs/1723145
commit 74aebed6dc13425233f2224668353cff7a112776 upstream.

Commit a50aac7193f1 introduces 'led.groups' and adds EKR support
for these groups. However, unlike the other devices with LEDs,
the EKR's LEDs are read-only and we shouldn't attempt to control
them in wacom_led_control().

See bug: https://sourceforge.net/p/linuxwacom/bugs/342/

Fixes: a50aac7193f1 ("HID: wacom: leds: dynamically allocate LED groups")
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>