target/microblaze: Use 'obj' in DEVICE() casts in mb_cpu_initfn()

target/microblaze: Use 'obj' in DEVICE() casts in mb_cpu_initfn()

We're about to make a change that removes the only other use
of the 'cpu' local variable in mb_cpu_initfn(); since the
DEVICE() casts work fine with the Object*, use that instead,
so that we can remove the local variable when we make the
following change.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250429132200.605611-2-peter.maydell@linaro.org