io/spi: mode numbers should have their own type
io/spi: mode numbers should have their own type

Aliasing int helps the discoverability of the precomputed mode numbers.

Change-Id: I4dd004641d28040227393a6a056dc45570211a5a
Reviewed-on: https://go-review.googlesource.com/21239
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

golang/exp,AkihiroSuda/golang-exp-aspectgo,golang/exp,AkihiroSuda/golang-exp-aspectgo,AkihiroSuda/golang-exp-aspectgo,golang/exp