leds: remove experimental flag from leds-clevo-mail

The leds-clevo-mail driver is in the mainline kernel since 2.6.25 and works
without severe problems. Make this driver available for a larger audience.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>