spi: rspi: Do not call rspi_receive_init() for TX-only

Since commit 8449fd76 ("spi: rspi: Merge
rspi_send_pio() and rspi_receive_pio()"), rspi_receive_init() is called
for transmit-only transfers too, while this is not needed.
Only call rspi_receive_init() when receiving, to preserve behavior on
RSPI on SH.

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