ahci: add a module parameter to ignore the SSS flags for async scanning

The SSS flag, which directs the OS to spin up one disk at a time
to not have the PSU blow out, sometimes gets set even when not needed.
The effect of this is a longer-than-needed boot time.

This patch adds a module parameter that makes the driver ignore SSS
at least as far as the parallel scan during boot is concerned...

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>