Some SAS HBAs don't want to go to the trouble of tracking port numbers,
so they'd simply like to say "add this port and give it a number".
This is especially beneficial from the hotplug point of view, since
tracking ports and the available number space can be a real pain.

The current implementation uses an incrementing number per expander to
add the port on.  However, since there can never be more ports than
there are phys, a later implementation will try to be more intelligent
about this.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>