revert available() algorithm (#914)

* fix #913

Essentially, this reverts the algorithm used to satisfy #650 as it has proved erroneous on fast MCUs.

RF24.cpp

@@ -1521,22 +1521,21 @@ uint8_t RF24::getDynamicPayloadSize(void)
 
 bool RF24::available(void)
 {
-    uint8_t pipe = 0;
+    uint8_t pipe = RF24_NO_FETCH_PIPE;
     return available(&pipe);
 }
 
 /****************************************************************************/
 
 bool RF24::available(uint8_t* pipe_num)
 {
-    // get implied RX FIFO empty flag from status byte
-    uint8_t pipe = (get_status() >> RX_P_NO) & 0x07;
-    if (pipe > 5)
+    if (read_register(FIFO_STATUS) & 1) { // if RX FIFO is empty
         return 0;
+    }
 
     // If the caller wants the pipe number, include that
-    if (pipe_num)
-        *pipe_num = pipe;
+    if (*pipe_num != RF24_NO_FETCH_PIPE)
+        *pipe_num = (get_status() >> RX_P_NO) & 0x07;
 
     return 1;
 }

RF24_config.h

@@ -44,6 +44,9 @@
     #define RF24_SPI_SPEED 10000000
 #endif
 
+/// A sentinel used to control fetching the pipe info in `RF24::available()`.
+#define RF24_NO_FETCH_PIPE 0XFF
+
 //ATXMega
 #if defined(__AVR_ATxmega64D3__) || defined(__AVR_ATxmega128D3__) || defined(__AVR_ATxmega192D3__) || defined(__AVR_ATxmega256D3__) || defined(__AVR_ATxmega384D3__)
     // In order to be available both in Windows and Linux this should take presence here.