ahci: use ports implemented map instead of num_ports

The AHCI driver was incorrectly using the Capabilities register NP (number
of ports) field to determine which ports to activate. This commit changes
it to correctly use the PORTS_IMPL register as a port map.

Signed-off-by: Richard Gibbs <richard.gibbs@calxeda.com>
Reviewed-by: Tom Rini <trini@ti.com>