spi: rspi: Extract rspi_common_transfer()

Extract the common parts of rspi_transfer_one(), rspi_rz_transfer_one(),
and qspi_transfer_out_in() into the new function rspi_common_transfer().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>