stand/ofw: dev can't be NULL here

dev can't be NULL here. ofw_common_parsedev is always called via
devparse (indirectly through dv_parsedev() calls there which call it
with the args unchanged). In the past, ofw_getdev could call us with
NULL pointer for the parse-only case, but that's now all handled inside
of devparse for simplicity.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38041

(cherry picked from commit d38d8a4c4e9a3c4667307ac2163007ff193040b0)