Merge pull request ARMmbed#2032 from ARMmbed/ws_enable_multi_csma

WS: Enable checking CCA twice on TX channel

Author: Jarkko Paso

source/6LoWPAN/ws/ws_common_defines.h

@@ -233,7 +233,7 @@ typedef struct ws_bs_ie {
 #define WS_TACK_MAX_MS 5
 
 // With FHSS we need to check CCA twice on TX channel
-#define WS_NUMBER_OF_CSMA_PERIODS  1
+#define WS_NUMBER_OF_CSMA_PERIODS  2
 // Interval between two CCA checks
 #define WS_CSMA_MULTI_CCA_INTERVAL 1000