platform/x86: mlx-platform: Add define for the negative bus

Add define for the negative bus ID in order to use it in case no hotplug
device is associated with the hotplug interrupt signal. In this case,
the signal will be handled by the mlxreg-hotplug driver, but no device
will be associated with the signal.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>