leds-lp55xx: code refactoring on selftest function

 LP5521 and LP5523 have a selftest function which is run via the sysfs.
 Use lp55xx driver data and R/W functions rather than lp5521/5523 private data
 and functions.
 Additionally, if-statements are changed for code simplicity.
 Unused functions, lp5521/5523_read() are removed.

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