hw/s390x/ipl: avoid taking address of fields in packed struct

Compiling with GCC 9 complains

hw/s390x/ipl.c: In function ‘s390_ipl_set_boot_menu’:
hw/s390x/ipl.c:256:25: warning: taking address of packed member of ‘struct QemuIplParameters’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  256 |     uint32_t *timeout = &ipl->qipl.boot_menu_timeout;
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

This local variable is only present to save a little bit of
typing when setting the field later. Get rid of this to avoid
the warning about unaligned accesses.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190329111104.17223-14-berrange@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>