ata: ahci: Loop over the actual number of ports, not the maximum

The loop in ahci_start_ports() is looping over the maximum number of
SCSI devices in the system, which can be larger than the amount of ports
a particular AHCI controller has. The extra looping isn't directly
harmful because the link_port_map bitmap won't have the bit set for a
nonexistent port, but it is wasteful. Replace the loop limit with the
port count of the AHCI controller instead.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Simon Glass <sjg@chromium.org>