HID: wacom: Status luminance properties should set brightness of all LEDs

The wacom driver has (deprecated) sysfs properties `status0_luminance`
and `status1_luminance` that are used to control the low- and high-
level brightness values (llv and hlv) of the status LEDs. These two
properties had an effect on /all/ of the status LEDs.

After our driver switched to exposing each status LED individually
through the LED class, this behavior changed. These controls started
having only a temporary effect on the currently-lit LED. If a trigger
changed the current LED, the driver would switch the brightness back
to the llv/hlv values stored per-LED. (The code's current behavior of
updating the "global" e.g. `wacom->led.llv` values has essentially no
effect because those values are only used at initialization time).

This commit restores the original behavior by ensuring these properties
update the per-LED brightness for all LEDs.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>