spi: rcar: Fix uninitialized variable warning

The transfer data length variable is set based on the desired access
size, without a default case. This results in a compiler warning, even
though the access size is always set to a supported value. Create a
default case to silence the warning.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mark Brown <broonie@linaro.org>