leds: ledtrig-transient: Add SPDX license identifiers

Replace GPL license statements with SPDX GPL-2.0 license identifiers
and correct the module license to GPLv2.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>