leds: leds-pcs9532 - Move i2c work to a workqueque

Apparently these might be called under atomic context,
and i2c operations may sleep. BUG found by
Ross Burton <ross@burtonini.com>

Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>