stand/ofw: Access the parsing routine more directly

We don't need to check if something is a ZFS device. Instead, if the
found device has a parse routine, call it. Otherwise, just copy the
path.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37343

(cherry picked from commit d16083815caf32dd117487c709b33669cef4af6f)