spi: rspi: Fix loopback mode for Dual/Quad SPI Transfers
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: Geert Uytterhoeven <geert+renesas@linux-m68k.org> Signed-off-by: Mark Brown <broonie@linaro.org>