Fix RF switching logic on rp2040-lora board.

[Mictronics]

Complementary-pin control logic is required on the rp2040-lora board since this board uses a RTC6603SP RF switch that only supports such logic.
See schematic https://www.waveshare.com/img/devkit/RP2040-LoRa-HF/RP2040-LoRa-HF-details-11.jpg
and https://files.waveshare.com/upload/c/c6/Datasheet-RTC6603SP-RichWave.pdf

• Pin 4 on RF switch is controlled as usual by DIO2 from SX1262
• Pin 6 on RF switch is controlled by GPIO17 from RP2040
• GPIO17 must be defined as SX126X_RXEN and can't be used as ordinary GPIO (e.g. BUTTON_PIN)
• GPIO18 pin is wired to SX1262 busy pin

This should solve EMI problems (wrong CRC, packet loss etc.) reported on Discord. The root cause for EMI was the SX1262 TX power was never reaching the antenna since the RF switch was not set in TX mode.