hw/intc/arm_gicv3: Check for !MEMTX_OK instead of MEMTX_ERROR

hw/intc/arm_gicv3: Check for !MEMTX_OK instead of MEMTX_ERROR

Quoting Peter Maydell:

 "These MEMTX_* aren't from the memory transaction
  API functions; they're just being used by gicd_readl() and
  friends as a way to indicate a success/failure so that the
  actual MemoryRegionOps read/write fns like gicv3_dist_read()
  can log a guest error."

We are going to introduce more MemTxResult bits, so it is
safer to check for !MEMTX_OK rather than MEMTX_ERROR.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>