ahci: fix PORTS_IMPL override

If PORTS_IMPL register is zero, ahci initialize it to full mask
corresponding to nr_ports in the CAP register.  hpriv->cap, which is
initialized at the end of the function, is incorrectly used as value
of CAP causing ahci to always override PORTS_IMPL to 0x1 if it's zero.
Fix it.

This fixes a bug where early ich6 ahci can only access the first port.

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