leds: el15203000: Correct headers (of*.h -> mod_devicetable.h)

There is no user of of*.h headers, but mod_devicetable.h.
Update header block accordingly.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Oleh Kravchenko <oleg@kaa.org.ua>
Signed-off-by: Pavel Machek <pavel@ucw.cz>