stand/ofw: Use strpbrk instead of two strchrs

No need to call strchr twice, when one call to strpbrk will do the
job.. Test booted with qemu-powerpc + mac99 successfully.
Minor style(9) tweaks as well.

Sponsored by: Netflix

(cherry picked from commit b60164c9f4d823b3e9fbcb733f92d181c704f415)