ahci: move host flags over to pi.private_data

Private pi.flags area is full and we need more private flags.  Move
host private flags over to pi.private_data.  During initialization,
these flags are copied to hpriv->flags.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>