SPI with chip select to ground
Most SPI devices use the CS as a framing signal as well as for bus sharing, so you can't get away with just tying it to GND. From a quick look at the datasheet this appears to be the case for the MCP23S17.

If you need to save pins, consider using the IC's I2C sibling MCP23017. If you need to save pins and absolutely must use the MCP23S17, you could probably contrive some circuit to set CS high (i.e. deasserted) when SCLK has been idle for a few microseconds at the end of a transaction. That would be a pretty ugly hack though, and it would be difficult not to violate timing requirements on the other end when it needs to assert CS at the start of the next transaction.