samples: led_ws2812: fix dma property in nucleo_f070rb.overlay

Since commit 718c77a4 ("dts: arm: stm32f0 soc serie has dma of type
V2bis"), the DMA DT cell of STM32F0 MCU has been fixed. It now has three
elements instead of five previously.

This patch fixes the dma property used in nucleo_f070rb.overlay
accordingly.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>