io/spi: don't enforce any opener arguments at the driver level

Updates golang/go#15774.

Change-Id: I85e340635a74f265902aab5ff1e593b75085fe7a
Reviewed-on: https://go-review.googlesource.com/23289
Reviewed-by: Johan Euphrosine <proppy@google.com>
Reviewed-by: Minux Ma <minux@golang.org>
4 files changed