iwlwifi: change spin_lock to spin_lock_irqsave

Use spin_lock_irqsave() in interrupt handler to disable interrupts locally
and provide the spinlock on SMP. This covers both interrupt and SMP
concurrency.

With this changes, also fix the sparse warning issues.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Acked-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>