leds: ns2: Use struct_size() in devm_kzalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes. Also, remove unnecessary
function sizeof_ns2_led_priv().

This code was detected with the help of Coccinelle and, audited and
fixed manually.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>