spi: rspi: Merge rspi_send_pio() and rspi_receive_pio()

rspi_send_pio() and rspi_receive_pio() are very similar:
  - the former only sends data, using TX Only Mode,
  - the latter sends and receives full duplex data to/from the hardware,
    but uses dummy transmit data.
Merge them into rspi_transfer_out_in(), now supporting full duplex if
needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>