In case anyone is interested. To make the PCF8575 work with i2cget and i2cset a little trick is needed.

In my board I have a PCF8575 only using the first eight ports, 4 outputs and 4 inputs.

To disable/clear the 4 outputs
i2cset -y 0 0x20 0xf0 0x00

To set the first output
i2cset -y 0 0x20 0xf1 0x00

To set all the outputs
i2cset -y 0 0x20 0xff 0x00

The trick is that the byte used in the "command" parameter of the i2cset command after the i2caddress is the first byte sent to the pcf8575.

Hope it helps.