leds: lp55xx: fix the sysfs read operation

According to a sysfs documentation(Documentation/filesystem/sysfs.txt),
scnprintf() should be used in a read operation method.
It guarantees safe buffer size(PAGE_SIZE) which is allocated by the sysfs.

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