virtio: fix sparse return void-valued expression warnings

drivers/virtio/virtio_pci.c:148:2: warning: returning void-valued expression
drivers/virtio/virtio_pci.c:155:2: warning: returning void-valued expression

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>