crypto/cipher: fix OCFB
crypto/cipher: fix OCFB

I messed up when reading the OCFB spec.

TBR=rsc

R=rsc
CC=golang-dev
http://codereview.appspot.com/3739042
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go