Use the recently added sdbus_write_data() to write multiple
bytes at once, instead of looping calling sdbus_write_byte().

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200814092346.21825-6-f4bug@amsat.org>