Change the failure mode in option parsing to silently bailing out of option

Change the failure mode in option parsing to silently bailing out of option
negotiation rather than rejecting the request.

Apple OpenFirmware 3.0f3 (the version in my iMac) adds trailing garbage to the
end of an otherwise valid request.  Without this change, the requests were
rejected which prevented me from booting.

Reviewed by:	obrien