emul: Add stub `release` impl. for emulated SPI driver

`release` is a mandatory method in the `struct spi_driver_api` API but
is not implemented in the SPI emulator. This can cause a test calling
`spi_release()` to segfault. Add a stub implementation that just returns
zero.

Signed-off-by: Tristan Honscheid <honscheid@google.com>