leds-lp55xx: use common device attribute driver function

 lp5521/5523_register_sysfs() are replaced with lp55xx common driver function,
 lp55xx_register_sysfs().
 Chip specific device attributes are configurable using 'dev_attr_group'.

 Error condition name is changed:
 use specific error condition, 'err_register_sysfs' rather than unclear name,
 'fail2'.

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