pc-bios/s390-ccw: Use reset_psw pointer instead of hard-coded null pointer

When compiling the s390-ccw bios with clang, it emits a warning like this:

 pc-bios/s390-ccw/jump2ipl.c:86:9: warning: indirection of non-volatile null
  pointer will be deleted, not trap [-Wnull-dereference]
     if (*((uint64_t *)0) & RESET_PSW_MASK) {
         ^~~~~~~~~~~~~~~~
 pc-bios/s390-ccw/jump2ipl.c:86:9: note: consider using __builtin_trap() or
  qualifying pointer with 'volatile'

We could add a "volatile" here to shut it up, but on the other hand,
we also have a pointer variable called "reset_psw" in this file already
that points to the PSW at address 0, so we can simply use that pointer
variable instead.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210423142440.582188-1-thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/jump2ipl.c

@@ -82,8 +82,8 @@ void jump_to_low_kernel(void)
         jump_to_IPL_code(KERN_IMAGE_START);
     }
 
-    /* Trying to get PSW at zero address */
-    if (*((uint64_t *)0) & RESET_PSW_MASK) {
+    /* Trying to get PSW at zero address (pointed to by reset_psw) */
+    if (*reset_psw & RESET_PSW_MASK) {
         /*
          * Surely nobody will try running directly from lowcore, so
          * let's use 0 as an indication that we want to load the reset