This patch clears the irqstatus register with the exact same events it 
has read from it. Since the read-write operation is not atomic, a new 
irqstatus bit could have been set in between these operations and would 
then be cleared accidentally.

Secondly, we now don't need any spin lock protection when 
scheduling/completing napi poll as the isr will not execute anymore (as 
we turn off all interrupts now).

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>