iwlagn: add a get_irq method to iwl_bus_ops and use it

In order to remove a few more dereference to priv->pdev that will be killed
[Asoon, there is now a method to get the IRQ number.

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