[nrf51822][gcc_arm][c++]: fix 'impossible constraint in asm' error, used by ble app

Author: xiongyihui

libraries/mbed/targets/hal/TARGET_NORDIC/TARGET_MCU_NRF51822/Lib/s110_nrf51822_7_0_0/s110_nrf51822_7.0.0_API/include/nrf_svc.h

@@ -16,7 +16,7 @@
   { \
     __asm( \
         "svc %0\n" \
-        "bx r14" : : "I" (number) : "r0" \
+        "bx r14" : : "I" ((uint32_t)number) : "r0" \
     ); \
   }
 #elif defined (__ICCARM__)