dm: spi: Correct handling of SPI chip selects in sandbox

This code was not updated when the chip select handling was adjusted. Fix
it to call the correct function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Heiko Schocher <hs@denx.de>