Fix error value returned by ofw_bus_gen_get_node().

By definition ofw_bus_get_node() should consistently return -1 when there
is no associated OF node.

MFC after: 4 weeks
Discussed with: nwhitehorn
Analyzed in: https://reviews.freebsd.org/D30761

(cherry picked from commit 3eae4e106ac7222364fc9dc8c3d35d4ad8c5293a)