HID: roccat-koneplus: vorrect mode of sysfs attr 'sensor'

Mode of sysfs attribute 'sensor' was wrongly set to writeonly.
Corrected this to readwrite.

Signed-off-by: Stefan Achatz <erazor_de@users.sourceforge.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>