hw/char/serial: Let compiler pick serial_mm_ops[] array length

No need to enforce the MemoryRegionOps array length.
We index by device_endian enum, the compiler will easily
pick the correct length. Besides, this allow further
adjustments in the device_endian enum itself.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20251224134644.85582-4-philmd@linaro.org>