AHCI: Move host activation code into ahci_host_activate()

Currently host activation done by calling either function
ahci_host_activate() or ata_host_activate(). Consolidate
the code by only calling ahci_host_activate() for all AHCI
devices.

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