spi: spidev_test: Add support for Octal mode data transfers

Add support for octal transfers using the -8/--octal command line
parameter.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200416101835.14573-3-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>