leds: leds-ns2: fix private driver data storage

dev_set_drvdata() can't be used to set the driver private data pointer.
This would overwrite the led classdev object previously registered by
led_classdev_register().

Note that despite this mistake, the driver work fine because led_dat and
led_dat->cdev are at the same memory address.

Signed-off-by: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>