Bugfix: Deadlocks due to RPi SPI mutex code

- SPI transfern function calls transfernb function so a deadlock was
occurring when a lock was attempted prior to the call
- Thread should be unlocked before returning if bcm2835_init() fails

utility/RPi/spi.cpp

@@ -10,6 +10,7 @@ SPI::SPI() {
 void SPI::begin( int busNo ) {
     spiNoInterrupts();
 	if (!bcm2835_init()){
+        spiInterrupts();
 		return;
 	}
 	

utility/RPi/spi.h

@@ -50,9 +50,7 @@ void SPI::transfernb(char* tbuf, char* rbuf, uint32_t len){
 
 void SPI::transfern(char* buf, uint32_t len)
 {
-    spiNoInterrupts();
     transfernb(buf, buf, len);
-    spiInterrupts();
 }
 /**
  * \endcond