SPI plugin: on some platforms SPI communication may fail or not be established

Currently the SPI communication is configured that the transfers (one transfer is a single read/write request) are to be separated by a chip select deselection. The CS handling is done by the underlying driver and is provided by the target platform or operating system.
The application can control the behaviour via the cs_change bit of the spi_ioc_transfer structure.

Recent tests showed that various drivers interpret this bit in different ways and not consequent.
This implementations results on some platforms in communication problems.