pci-assign: propagate errors from assign_device()

Also, change the return type to "void"; the function is static (with a
sole caller) and the negative errno values are not distinguished from each
other.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
