HID: wacom: Initialize brightness of LED trigger

If an LED has a default_trigger set prior to being registered with
the subsystem, that trigger will be executed with a brightness value
defined by `trigger->brightness`. Our driver was not setting this
value, which was causing problems. It would cause the selected LED
to be turned off, as well as corrupt the hlv/llv values assigned to
other LEDs (since calling `wacom_led_brightness_set` will overite
these values).

This patch sets the value of `trigger->brightness` to an appropriate
value. We use `wacom_leds_brightness_get` to transform the llv/hlv
values into a brightness that is understood by the rest of the LED
subsystem.

Fixes: 822c91e7 ("leds: trigger: Store brightness set by led_trigger_event()")
Cc: stable@vger.kernel.org # v6.10+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>