dm: spi: Return 0 if driver does not implement ops->cs_info

If an SPI controller driver does not implement ops->cs_info, that
probably means any chip select number could be valid, hence let's
return 0 for spi_cs_info().

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> # SoPine
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>