armv8-m/arm_svcall.c: Fix compiler warning

regs[REG_R0] is uint32_t type, but '%d' expects int type.

Signed-off-by: Michael Jung <mijung@gmx.net>

arch/arm/src/armv8-m/arm_svcall.c

@@ -453,7 +453,7 @@ int arm_svcall(int irq, FAR void *context, FAR void *arg)
 
           rtcb->flags         |= TCB_FLAG_SYSCALL;
 #else
-          svcerr("ERROR: Bad SYS call: %d\n", regs[REG_R0]);
+          svcerr("ERROR: Bad SYS call: %d\n", (int)regs[REG_R0]);
 #endif
         }
         break;