leds: core: Use EXPORT_SYMBOL_GPL consistently

LED core has a mixture of EXPORT_SYMBOL and EXPORT_SYMBOL_GPL macros.
This patch fixes this discrepancy and switches to using EXPORT_SYMBOL_GPL
for each exported function.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Pavel Machek <pavel@ucw.cz>