SPI Chip Select behavior:

With the Blackfin on-chip SPI peripheral, there is some logic tied to the CPHA
bit whether the Slave Select Line is controlled by hardware (CPHA=0) or
controlled by software (CPHA=1). However, the Linux SPI bus driver assumes that
the Slave Select is always under software control and being asserted during
the entire SPI transfer. - And not just bits_per_word duration.

In most cases you can utilize SPI MODE_3 instead of MODE_0 to work-around this
behavior. If your SPI slave device in question requires SPI MODE_0 or MODE_2
timing, you can utilize the GPIO controlled SPI Slave Select option instead.

You can even use the same pin whose peripheral role is a SSEL,
but use it as a GPIO instead.