[SX127x] Set default FSK BT shaping to 0.5 for SX126x FSK (#123)

examples/SX127x/SX127x_FSK_Modem/SX127x_FSK_Modem.ino

@@ -38,7 +38,7 @@ void setup() {
   // Rx bandwidth:                125.0 kHz
   // output power:                13 dBm
   // current limit:               100 mA
-  // data shaping:                Gaussian, BT = 0.3
+  // data shaping:                Gaussian, BT = 0.5
   // sync word:                   0x2D  0x01
   // OOK modulation:              disabled
   int state = fsk.beginFSK();

src/modules/SX127x/SX1272.cpp

@@ -413,6 +413,10 @@ int16_t SX1272::configFSK() {
 
   // set fast PLL hop
   state = _mod->SPIsetRegValue(SX1272_REG_PLL_HOP, SX127X_FAST_HOP_ON, 7, 7);
+  RADIOLIB_ASSERT(state);
+
+  // set Gauss filter BT product to 0.5
+  state = _mod->SPIsetRegValue(SX127X_REG_OP_MODE, SX1272_FSK_GAUSSIAN_0_5, 4, 3);
 
   return(state);
 }

src/modules/SX127x/SX1278.cpp

@@ -491,6 +491,10 @@ int16_t SX1278::configFSK() {
 
   // set fast PLL hop
   state = _mod->SPIsetRegValue(SX1278_REG_PLL_HOP, SX127X_FAST_HOP_ON, 7, 7);
+  RADIOLIB_ASSERT(state);
+
+  // set Gauss filter BT product to 0.5
+  state = _mod->SPIsetRegValue(SX127X_REG_PA_RAMP, SX1278_FSK_GAUSSIAN_0_5, 6, 5);
 
   return(state);
 }