sysbus: Remove ignored return value of FindSysbusDeviceFunc

sysbus: Remove ignored return value of FindSysbusDeviceFunc

Functions of type FindSysbusDeviceFunc currently return an integer.
However, this return value is always ignored by the caller in
find_sysbus_device().

This changes the function type to return void, to avoid confusion over
the function semantics.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>