AHCI: Do not acquire ata_host::lock from single IRQ handler

There is no need to acquire ata_host::lock spinlock from
hardware context single IRQ interrupt handler since the
handler does not access host data that could be altered
by concurrent processors.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: linux-ide@vger.kernel.org