leds: leds-ns2: fix oops at module removal

This patch fixes a regression introduced by commit 72052fcc10
("leds: leds-ns2: add device tree binding").

When the driver is initialized with device tree data, platform_data
pointer is NULL. This causes a kernel oops at removal.

To fix this bug, num_leds is moved into driver_data and platform_data
is not longer used from ns2_led_remove().

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>