spi: rspi: Remove unused variable in rspi_rz_transfer_one()

Introduced by commit 8b983e90 ("spi: rspi:
Extract rspi_common_transfer()"), which removed its users.

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