spibus: Make ofw_spibus probe just a little more favored

With ade70a1ad(svn r332196) ofw_spibus probes at the BUS_PROBE_DEFAULT
instead of 0. However, this races with spibus, resulting in ofw_spibus
often times losing the race and the OFW node not being referenced. This
in turn causes child device tree nodes to not be attached. Solve this
by returning 1 higher than spibus, just like acpi_spibus.

Sponsored by: Juniper Networks, Inc.
MFC after: 1 week