drm/rockchip: dsi: make hstt_table static

This symbol is not used outside of dw-mipi-dsi-rockchip.c, so marks
it static.

Fix the following sparse warning:

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c:646:13: warning: symbol
'hstt_table' was not declared. Should it be static?
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1628218664-14230-1-git-send-email-jiapeng.chong@linux.alibaba.com

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c

@@ -642,7 +642,7 @@ struct hstt {
 }
 
 /* Table A-3 High-Speed Transition Times */
-struct hstt hstt_table[] = {
+static struct hstt hstt_table[] = {
 	HSTT(  90,  32, 20,  26, 13),
 	HSTT( 100,  35, 23,  28, 14),
 	HSTT( 110,  32, 22,  26, 13),