iwlwifi: clear unattended interrupts in tasklet

Previously in interrupt handling tasklet, iwlwifi driver only clear/ack
those interrupts that are enabled by the driver through inta_mask.
If the hardware generates unattended interrupts, driver will not ack them,
defeating the interrupt coalescing feature. This results in high number
of interrupts per second and high CPU utilization.

This patch addresses this issue by acking those unattended interrupts
in the tasklet. Local test showed an order of magnitude improvement
in terms of the number of interrupts without sacrificing networking
throughput. This is a workaround for hardware issue.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>