Fix RAK3401 SKY66122-11 FEM control: enable CSD/CPS for proper PA and LNA operation

The RAK13302 1W module uses a Skyworks SKY66122-11 front-end module with
three digital control pins (CSD, CTX, CPS) that must be actively driven
by the host MCU. The previous code only managed CTX (GPIO 31) — toggling
it for TX/RX — but never initialized CSD (GPIO 24) or CPS (GPIO 21),
leaving them floating with no pull-up/pull-down resistors on the PCB.

With floating CSD and CPS, the SKY66122 was in an undefined operating
mode:
- The 30 dB TX PA may not have been reliably engaging
- The 16 dB RX LNA was never reliably active, degrading receive
sensitivity

variants/rak3401/RAK3401Board.h

@@ -38,13 +38,14 @@ public:
     return "RAK 3401";
   }
 
-#ifdef P_LORA_PA_EN
+  // SKY66122 FEM TX/RX switching via CTX pin.
+  // CTX=HIGH: TX mode + 5V boost ON (PA powered from VCC1/VCC2)
+  // CTX=LOW:  RX mode + 5V boost OFF (LNA powered from VSUP1 at 3.3V)
   void onBeforeTransmit() override {
-    digitalWrite(P_LORA_PA_EN, HIGH);  // Enable PA before transmission
+    digitalWrite(P_LORA_PA_EN, HIGH);  // CTX=1: TX mode, boost on
   }
 
   void onAfterTransmit() override {
-    digitalWrite(P_LORA_PA_EN, LOW);   // Disable PA after transmission to save power
+    digitalWrite(P_LORA_PA_EN, LOW);   // CTX=0: RX mode, boost off
   }
-#endif
 };