sam: Optimized delayMicroseconds() (Rob Tillaart)

See arduino#1121

Author: cmaglie

cores/arduino/wiring.h

@@ -64,11 +64,12 @@ extern void delay( uint32_t dwMs ) ;
  */
 static inline void delayMicroseconds(uint32_t) __attribute__((always_inline, unused));
 static inline void delayMicroseconds(uint32_t usec){
+    if (usec == 0) return;
     uint32_t n = usec * (VARIANT_MCK / 3000000);
     asm volatile(
         "L_%=_delayMicroseconds:"       "\n\t"
         "subs   %0, #1"                 "\n\t"
-        "bge    L_%=_delayMicroseconds" "\n"
+        "bne    L_%=_delayMicroseconds" "\n"
         : "+r" (n) :
     );
 }