leds-lp5523: Fix riskiness of the page fault

The last attribute should be terminated as NULL because any member of
attribute structure is accessed while adding the sysfs file.
If not, invalid address may cause the page fault problem.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>