spi: dw: Convert dw_spi::num_cs to u32

spi: dw: Convert dw_spi::num_cs to u32

Number of native chip-select lines is either retrieved from the "num-cs"
DT-property or auto-detected in the generic DW APB/AHB SSI probe method.
In the former case the property is supposed to be of the "u32" size.
Convert the field type to being u32 then to be able to drop the temporary
variable afterwards.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240424150657.9678-4-fancer.lancer@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>