io/spi: namespace devfs-related constants
io/spi: namespace devfs-related constants

Change-Id: I5c2dc756cec4e0e1082736c58371ced6f1844a24
Reviewed-on: https://go-review.googlesource.com/21383
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

golang/exp,golang/exp,golang/exp