leds: lp55xx: Fix file permissions to use DEVICE_ATTR macros

Fix the checkpatch warnings for the use of the file permission macros.
In converting the file permissions to the DEVICE_ATTR_XX macros the
call back function names needed to be updated within the code.

This means that the lp55xx_ needed to be dropped in the name to keep in
harmony with the ABI documentation.

Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>