While normal Dual and Quad SPI Transfers are unidirectional, we must do
a bidirectional transfer if loopback mode is enabled, else rx_buf is not
filled.

With spidev it seemed to work, as spidev uses the same buffer for
tranmission and reception.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: NMark Brown <broonie@linaro.org>