hw/char/sifive_uart: Fix broken UART on big endian hosts

hw/char/sifive_uart: Fix broken UART on big endian hosts

Casting a "uint32_t *" to a "uint8_t *" to get to the lowest 8-bit
part of the value does not work on big endian hosts. We've got to
take the proper detour through an 8-bit variable.

Fixes: 53c1557b23 ("hw/char: sifive_uart: Print uart characters async")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20241104163504.305955-1-thuth@redhat.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>