ark_fpv: fix DMA allocation

[dakejahl]

Problem
When bidirectional dshot is enabled the RC RX stops working.

Solution
Remove USART3 (debug port) RX/TX DMA. I don't understand why this fixes it though, since it is being assigned to DMA2.

Other updates

• add USART6 (RC) TX DMA for CRSF Telemetry
• remove UART2 (VTX) TX DMA since that signal is unused
• update debug prints in dshot

@dagar do you have any ideas about why this might be happening?

[github-actions]

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 208 byte (0.01 %)]

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px4_fmu-v6x [Total VM Diff: 208 byte (0.01 %)]

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--------------  -------------- 
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Updated: 2025-06-04T22:17:09