hw/alpha/typhoon: Stop calling cpu_unassigned_access()

hw/alpha/typhoon: Stop calling cpu_unassigned_access()

The typhoon MemoryRegionOps callbacks directly call
cpu_unassigned_access(), presumably as the old-fashioned way
to provoke a CPU exception.  This won't work since commit
6ad4d7eed05a1e235 when we switched Alpha over to the
transaction_failed hook API, because now cpu_unassigned_access()
is a no-op for Alpha.

Make the MemoryRegionOps callbacks use the read_with_attrs
and write_with_attrs hooks, so they can signal a failure
that should cause a CPU exception by returning MEMTX_ERROR.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20181210173350.13073-1-peter.maydell@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>