libata: allocate ap separately from shost

Don't embed ap inside shost.  Allocate it separately and point it back
from shosts's hostdata.  This makes port allocation more flexible and
allows regular ATA and SAS share host alloc/init paths.

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