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: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>