Reading data from potentiometer with MISO and MOSI on same pin
You need to do a little conversion:


Connect the SDI/SDO line to MISO on the Arduino.
Connect a 1kΩ resistor between MISO and MOSI on the Arduino.


Then you first send the command byte(s) to the slave. You will receive these back, but you can just ignore them.

Then you receive the data. You do this by sending 0xFF for each byte. The 1kΩ resistor means the slave can override the 0xFF from the MOSI. The MOSI -> MISO resistor effectively becomes a pullup resistor and is, to all intents and purposes, ignored (it's just in parallel to the pullup resistor internal to the slave).