ahci: switch from 'threaded' to 'hardirq' interrupt handling

For high frequency I/O the overhead of threaded interrupts impacts
performance.  A quick out-of-the-box test (i.e. no affinity tuning)
shows ~10% random read performance at ~20% less cpu.  The cpu wins
appear to be from reduced lock contention.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>