spi: dw: Set default value if reg-io-width isn't specified

According to the dt-bindings, the default value of reg-io-width is 4.
However, the value becomes zero when reg-io-width isn't specified.

Should set the actual value to dws->reg_io_width, considering it
referenced.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20230807001621.196776-1-hayashi.kunihiko@socionext.com


Signed-off-by: Mark Brown <broonie@kernel.org>