spi: Kill superfluous cast in spi_w8r16()

spi_write_then_read() takes a "void *" for rxbuf, so there's no need to
cast the buffer pointer to "u8 *".

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