iwlagn: store the irq number to iwl_bus just like dev

Since the irq number is just an unsigned int, store it inside iwl_bus
instead of calling the get_irq ops every time it is needed.

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