Use the correct type for i when iterating over buf to avoid unlikely
negative array indexing in iicrdwr(..)

Differential Revision: https://reviews.freebsd.org/D5132
Obtained from: HardenedBSD
PR: 206754
Reported by: CTurt <cturt@hardenedbsd.org>
Submitted by: Madhi Moktari <mokhi64@gmail.com>
Sponsored by: EMC / Isilon Storage Division