spi: core: Ignore unsupported spi-[tr]x-bus-width property values

Rejecting unsupported values of spi-tx-bus-width and spi-rx-bus-width
may break compatibility with future DTs. Just ignore them, falling back
to Single SPI Transfers.

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