spi: rspi: Fix loopback mode for Dual/Quad SPI Transfers

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>
(cherry picked from commit ba824d4971691a7f1f66429e378a08a95fbb5b79)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>