I see that you are trying to send data from the slave since you load the transmit register
SPI_S_REG_VAL(DR) = 0xD8; // write some sample data to the tx buffer
This data can not be sent since MISO is defined as an input instead of an output.
Also since this is a slave SPI the data will not be transmitted by loading the DR register. This data is only held there until the Master SPI selects the slave by lowering SS and the master must send data to the slave in order to generate the 8 clock cycles that will cause the data in the slave SPI to be transmitted to the master.