fix(waveshare-rp2040): add SX126X_RXEN=17 to restore RX sensitivity

The Waveshare RP2040-LoRa board has an RF switch with two complementary
control lines:
- DIO2 (CTRL)  -- driven automatically by the SX1262 chip: HIGH on TX,
  LOW on RX
- GPIO17 (!CTRL) -- was not configured in MeshCore, so it floated and
  the switch never properly selected the LNA/RX path

Without this pin driven, the RF switch never switches to RX mode,
resulting in completely degraded reception. TX was unaffected because
DIO2 alone is sufficient to activate the TX path.

Adding SX126X_RXEN=17 lets RadioLib drive GPIO17 as the complement of
DIO2, so the switch correctly routes the signal to the LNA on receive.

Reference: https://files.waveshare.com/wiki/RP2040-LoRa/Rp2040-lora-sch.pdf

variants/waveshare_rp2040_lora/platformio.ini

@@ -20,6 +20,7 @@ build_flags = ${rp2040_base.build_flags}
   -D P_LORA_MOSI=15
   -D P_LORA_TX_LED=25
   -D SX126X_DIO2_AS_RF_SWITCH=true
+  -D SX126X_RXEN=17
   -D SX126X_DIO3_TCXO_VOLTAGE=1.8
   -D SX126X_RX_BOOSTED_GAIN=1
   -D LORA_TX_POWER=22