memory: don't try to adjust endianness for zero length eventfd

There's no need to adjust endianness for zero length eventfd since the
data wrote was actually ignored by kernel. So skip the adjust in this
case to fix a possible crash when trying to use wildcard mmio eventfd
in ppc.

Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>