drm/panel: st7703: Rename CMD_UNKNOWN_C6 to CMD_SETECO

A later revision of the datasheet for the ST7703 refers to this command
as "SETECO".

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230426143213.4178586-3-macroalpha82@gmail.com