leds: leds-ns2: fix locking

This patch replace all the lock functions with the irq safe variant.
The ns2_led_{set,get}_mode() functions must be safe in all context.

For example, the trigger timer call led_set_brightness() in a softirq
context.

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