spi: spi-mem: Fix Dual/Quad modes on Octal-capable devices

Currently buswidths 2 and 4 are rejected for a device that advertises
Octal capabilities.  Allow these buswidths, just like is done for
buswidth 2 and Quad-capable devices.

Fixes: b12a084c

 ("spi: spi-mem: add support for octal mode I/O data transfer")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200416101418.14379-1-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>