spi: spidev: Add support for Octal mode data transfers

Include the flags for Octal mode data transfers in the mask, so
userspace can set them.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200416101835.14573-2-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>