leds: lt3593: merge functions and clean up code

In preparation to DT probe functionality, merge create_lt3593_led() into
its only call-site. The DT based setup code will be quite different, so this
internal helper function is of no help.

This also changes the way the driver works by only handling one entry inside
'struct gpio_led_platform_data'. If multiple devices of the same type are
found in a design, there should be a platform device for each of them. The
only mainline user of this driver is not affected by this change.

Last, use devm_led_classdev_register() instead of
led_classdev_register(), so the driver's remove callback can go away.

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