ahci: move port_map handling to ahci_save_initial_config()

Move cross checking between port_map and cap.n_ports into
ahci_save_initial_config().  After save_initial_config is done,
hpriv->port_map is always setup properly.

Tested on JMB363, ICH7 and ICH8 (with dummy ports).

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