Added setGain for LNA Gain (#408)

* Added setGain for LNA Gain

Added setGain for LNA Gain

* Update API.md

Added API for setGain

* Added example to use setGain

Just use setGain after LoRa.begin
// set maximum LNA for better RX sensitivity
// 0: ADC is used and LNA gain is not used.
// 1-6: ADC is not used and LNA gain is used.
LoRa.setGain(6);

* Fixed spacing only

To change tabs to spaces.

* Delete LoRaReceiverCallbackWithLNAGain.ino

To remove as unnecessary.

* To add example for setGain

To add example for setGain as an optional setting.

* Added setGain

To add setGain

* Fixed typo for AGC

Fixed typo for AGC

* Fixed comment for LoRa.setGain(6)

* Make comment for setGain simpler

Make comment for setGain simpler

src/LoRa.cpp

@@ -607,6 +607,32 @@ void LoRaClass::setOCP(uint8_t mA)
   writeRegister(REG_OCP, 0x20 | (0x1F & ocpTrim));
 }
 
+void LoRaClass::setGain(uint8_t gain)
+{
+  // check allowed range
+  if (gain > 6) {
+    gain = 6;
+  }
+  
+  // set to standby
+  idle();
+  
+  // set gain
+  if (gain == 0) {
+    // if gain = 0, enable AGC
+    writeRegister(REG_MODEM_CONFIG_3, 0x04);
+  } else {
+    // disable AGC
+    writeRegister(REG_MODEM_CONFIG_3, 0x00);
+	
+    // clear Gain and set LNA boost
+    writeRegister(REG_LNA, 0x03);
+	
+    // set gain
+    writeRegister(REG_LNA, readRegister(REG_LNA) | (gain << 5));
+  }
+}
+
 byte LoRaClass::random()
 {
   return readRegister(REG_RSSI_WIDEBAND);

src/LoRa.h

@@ -77,6 +77,8 @@ public:
   void disableInvertIQ();
   
   void setOCP(uint8_t mA); // Over Current Protection control
+  
+  void setGain(uint8_t gain); // Set LNA gain
 
   // deprecated
   void crc() { enableCrc(); }