spi: core: Use spi_sync_transfer() in spi_write()/spi_read()

Simplify spi_write() and spi_read() using the spi_sync_transfer()
helper.

This requires moving spi_sync_transfer() up.

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