leds: lt3593: update email address and switch to SPDX license header

Update the email address in the module information and in the comments
and in the module information, and drop the existing boilerplate in
favor of a SPDX header.

Also sync the module license information with the header.

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