leds: powernv: Implement brightness_set_blocking op

Since brightness setting can sleep for this driver, implement
brightness_set_blocking op, instead of brightness_set.
It makes this driver compatible with LED triggers.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>