drm/panel: novatek-nt35560: Fix invalid return value

Fix bug in nt35560_set_brightness() which causes the function to
erroneously report an error. mipi_dsi_dcs_write() returns either a
negative value when an error occurred or a positive number of bytes
written when no error occurred. The buggy code reports an error under
either condition.

Fixes: 8152c2bf ("drm/panel: Add driver for Sony ACX424AKP panel")
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Brigham Campbell <me@brighamcampbell.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250731032343.1258366-2-me@brighamcampbell.com