Minor optimisation to mcr20a-rf-driver code

This is a minor optimisation to the mcr20a-rf-driver code:

Note that function parameter is pointer. The code is more readable when 'NULL' instead of using '0'.

Author: DavidLin1577

components/802.15.4_RF/mcr20a-rf-driver/source/MCR20Drv.c

@@ -45,6 +45,7 @@
 #if defined(MBED_CONF_NANOSTACK_CONFIGURATION) && DEVICE_SPI && DEVICE_INTERRUPTIN && defined(MBED_CONF_RTOS_PRESENT)
 
 #include "platform/mbed_critical.h"
+#include <string.h>
 
 /*****************************************************************************
 *                               PRIVATE VARIABLES                           *