irq: smp_affinity: Initialize work struct only once

The work function to handle the irq affinity change is currently being
set from setup_affinity, which is called whenever the affinity changes.
Initialize the work function only once when the irq desc object's
defaults are set up.

CRs-Fixed: 756463
Change-Id: I66732f8c01cba166c41ce89c329d313eeaea8a7d
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>