leds: lt3593: drop pdata handling code

The only user of this driver in mainline has now moved to DTS, so the pdata
code is no longer in use by anyone. Let's drop some dead code, and make the
driver depend on CONFIG_OF.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>