Fix ROM check for callbacks

Check now works if ROMLimit is precisely at the end
of ROM causing ROMLimit to not be a valid immediate.

s/Kernel

@@ -894,8 +894,8 @@ callback_checking
         CMP     r0, #ROM
         BHS     Do_CallBack
  |
-        CMP     r0, #ROM
-        RSBHIS  r12, r0, #ROMLimit
+        SUBS    r12, r0, #ROM
+        RSBHIS  r12, r12, #ROMLimit - ROM
         BHI     Do_CallBack
  ]
         ORR     r11, r11, #CBack_Postpone      ; signal to IRQs