oce: Tighten input validation in the SIOCGI2C handler.

Missing validation meant that it was possible to read 8 bytes beyond
the end of sfp_vpd_dump_buffer.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: delphij, ram
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22859