leds: leds-ns2: handle can_sleep GPIOs

leds: leds-ns2: handle can_sleep GPIOs

On the board n090401 (Seagate NAS 4-Bay), some of the LEDs are handled
by the leds-ns2 driver. This LEDs are connected to an I2C GPIO expander
(PCA95554PW) which means that GPIO access may sleep. This patch makes
leds-ns2 compatible with such GPIOs by using the *_cansleep() variant of
the GPIO functions. As a drawback this functions can't be used safely in
a timer context (with the timer LED trigger for example). To fix this
issue, a workqueue mechanism (copied from the leds-gpio driver) is used.

Note that this patch also updates slightly the ns2_led_sata_store
function. The LED state is now retrieved from cached values instead of
reading the GPIOs previously. This prevents ns2_led_sata_store from
working with a stale LED state (which may happen when a delayed work
is pending).

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Vincent Donnefort <vdonnefort@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>