AHCI: Move ahci_host_activate() function to libahci.c

This update is a prerequisite for consolidation of
AHCI host activation code within ahci_host_activate()
function.

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