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

This reverts commit 33fb0d01.

18dcf433 ("AHCI: Optimize single IRQ interrupt processing") is
scheduled to be reverted.  This is an optimization dependent on the
mentioned commit.  Revert it first.

Signed-off-by: Tejun Heo <tj@kernel.org>