[SX127x] Clarify FSK FIFO size is due to hardware (#1578)

src/modules/SX127x/SX127x.h

@@ -12,7 +12,7 @@
 // SX127x physical layer properties
 #define RADIOLIB_SX127X_FREQUENCY_STEP_SIZE                     61.03515625
 #define RADIOLIB_SX127X_MAX_PACKET_LENGTH                       255
-#define RADIOLIB_SX127X_MAX_PACKET_LENGTH_FSK                   64
+#define RADIOLIB_SX127X_MAX_PACKET_LENGTH_FSK                   64 // as per datasheet Rev. 7, page 66, the FSK FIFO is just 64 bytes
 #define RADIOLIB_SX127X_CRYSTAL_FREQ                            32.0f
 #define RADIOLIB_SX127X_DIV_EXPONENT                            19