SPI bus speed divider to 4, suggested by Greg Copeland. Also removed...

SPI bus speed divider to 4, suggested by Greg Copeland.  Also removed duplicated spi setup out of begin(), as it's all in csn()

RF24.cpp

@@ -28,9 +28,13 @@
 
 void RF24::csn(int mode)
 {
+  // Minimum ideal SPI bus speed is 2x data rate
+  // If we assume 2Mbs data rate and 16Mhz clock, a
+  // divider of 4 is the minimum we want.
+  // CLK:BUS 8Mhz:2Mhz, 16Mhz:4Mhz, or 20Mhz:5Mhz
   SPI.setBitOrder(MSBFIRST);
   SPI.setDataMode(SPI_MODE0);
-  SPI.setClockDivider(SPI_CLOCK_DIV2);
+  SPI.setClockDivider(SPI_CLOCK_DIV4);
   digitalWrite(csn_pin,mode);
 }
 
@@ -315,17 +319,7 @@ void RF24::begin(void)
   pinMode(csn_pin,OUTPUT);
 
   // Initialize SPI bus
-  // Minimum ideal SPI bus speed is 2x data rate
-  // If we assume 2Mbs data rate and 16Mhz clock, a
-  // divider of 4 is the minimum we want.
-  // CLK:BUS 8Mhz:2Mhz, 16Mhz:4Mhz, or 20Mhz:5Mhz
-  // We'll use a divider of 2 which will work up to
-  // MCU speeds of 20Mhz.
-  // CLK:BUS 8Mhz:4Mhz, 16Mhz:8Mhz, or 20Mhz:10Mhz (max)
   SPI.begin();
-  SPI.setBitOrder(MSBFIRST);
-  SPI.setDataMode(SPI_MODE0);
-  SPI.setClockDivider(SPI_CLOCK_DIV2);
 
   ce(LOW);
   csn(HIGH);