sim: fix compile warning

sim/sim_doirq.c: In function 'sim_doirq':
Error: sim/sim_doirq.c:79:10: error: function may return address of local variable [-Werror=return-local-addr]
   79 |   return regs;
      |          ^~~~
sim/sim_doirq.c:44:14: note: declared here
   44 |   xcpt_reg_t tmp[XCPTCONTEXT_REGS];
      |              ^~~

Signed-off-by: ligd <liguiding1@xiaomi.com>

arch/sim/src/sim/sim_doirq.c

@@ -76,5 +76,5 @@ void *sim_doirq(int irq, void *context)
       sim_fullcontextrestore(regs);
     }
 
-  return regs;
+  return NULL;
 }