bytes: make all readOp constants actually typed

bytes: make all readOp constants actually typed

This is a regression introduced in golang.org/cl/28817. That change got
rid of the iota, which meant that the type was no longer applied to all
the constant names.

Re-add the iota starting at -1, simplifying the code and adding the
types once more.

Change-Id: I38bd0e04f8d298196bccd33651e29f5011401a8d
Reviewed-on: https://go-review.googlesource.com/78617
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>