target/s390x/mmu: Replace [cpu_physical_memory -> address_space]_rw()

target/s390x/mmu: Replace [cpu_physical_memory -> address_space]_rw()

When cpu_address_space_init() isn't called during vCPU creation,
its single address space is the global &address_space_memory.

As s390x boards don't call cpu_address_space_init(), cpu->as
points to &address_space_memory.

We can then replace cpu_physical_memory_rw() by the semantically
equivalent address_space_rw() call.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20251002084203.63899-9-philmd@linaro.org>