ahci: add another PCI ID for marvell

1b4b:91a3 seems to be another PCI ID for marvell ahci.  Add it.
Reported and tested in the following thread.

 http://thread.gmane.org/gmane.linux.kernel/1068354



Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Borislav Petkov <bp@alien8.de>
Reported-by: Alessandro Tagliapietra <tagliapietra.alessandro@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>