newbus: Remove redundant check for 0

We already checked to see if the return code is 0 above. This code is
redundant. However, it's here so we can break out two layers, which a
simple goto fixes. A subsequent change makes the pri < 0 condition no
longer hold. This allows us to simplify a few things.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D44269