iwlagn: move sync_irq to transport layer

Since all the irq / tasklet is now handled in the transport layer, it should
give an API to ensure that all the irq / tasklet have finished running. This
will allow the upper layer to release all its resources.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>