hw/block/pflash_cfi02: Set romd mode in pflash_cfi02_realize()

The ROMD mode isn't related to mapping setup.
Ideally we'd set this mode when the state machine resets,
but for now simply move it to pflash_cfi02_realize() to
not introduce logical change.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210325120921.858993-2-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>