MAC unit tests: updated cca prepare test

Author: Jarkko Paso

test/nanostack/unittest/mac/mac_pd_sap/test_mac_pd_sap.c

@@ -829,7 +829,7 @@ bool test_mac_cca_prepare_cb()
 
     memset(&test_packet, 0, sizeof(mac_pre_build_frame_t));
     rf_ptr.active_pd_data_request = &test_packet;
-    test_packet.csma_periods_left = 1;
+    test_packet.csma_periods_left = 0;
     test_packet.fcf_dsn.frametype = FC_BEACON_FRAME;
     test_packet.asynch_request = false;
     fhss_callback_response = -1;