spi: Don't decode data lines if CS isn't asserted

Avoid decoding and outputting data from the SPI bus if the CS pin
hasn't been asserted. This avoids confusing both users and stacked
decoders which otherwise end up seeing traffic intended for other
chips (or just noise).

Note: The old behavior of decoding all traffic is still in place if
no CS pin has been wired up to the decoder.

This fixes bug #559.