io/spi: rename Transfer to Tx

To keep the naming consistent with the I2C package. (see CL/23405)

Change-Id: I8e2e51a5eef70cf6b84bc0496571c405e544b02f
Reviewed-on: https://go-review.googlesource.com/23520
Reviewed-by: Minux Ma <minux@golang.org>
4 files changed