sata_nv: MSI support, disabled by default

At least the nVidia MCP55 controller quite happily supports MSI.
This adds an option to use it. It is disabled by default.
As per feedback by Robert Hancock, it will honour the user
request as the kernel will not enable MSI where the controller
or the specific system configuration do not support it.

Signed-off-by: Tony Vroon <tony@linx.net>
Cc: Robert Hancock <hancockrwd@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>