device_get_path(): do not drop the error from BUS_GET_DEVICE_PATH()

Later it would silently converted to ENOMEM always, because any error
was reported as NULL return path.

Reviewed by: jhb, takawata
Discussed with: imp
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36899