drivers: dw1000: fix SPI GPIO CS

Since commit 5963ebaf
("drivers: spi: CS configuration through devicetree")
the SPI GPIO CS flags are obtained from DT,
but the patch series has missed the necessary changes
for the ieee802154_dw1000 driver and decawave_dwm1001_dev board.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>