leds-atmel-pwm.c: Make pwmled_probe() __devinit

Commit 892a8843 (leds: convert led platform drivers to module_platform_driver)
is omitting the section mismatch error: so change annotation of the probe
function to __devinit instead of __init.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>