drm/bridge/sii8620: Fix memory corruption

Function sii8620_mt_read_devcap_reg_recv() used to read array index
from a wrong msg register, which caused writing out of array
bounds. It led to writing on other fields of struct sii8620.
Signed-off-by: Maciej Purski <m.purski@samsung.com>
Fixes: e9c6da27 ("drm/bridge/sii8620: add reading device capability
       registers")
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1503311571-25819-1-git-send-email-m.purski@samsung.com

drivers/gpu/drm/bridge/sil-sii8620.c

@@ -597,9 +597,9 @@ static void sii8620_mt_read_devcap(struct sii8620 *ctx, bool xdevcap)
 static void sii8620_mt_read_devcap_reg_recv(struct sii8620 *ctx,
 		struct sii8620_mt_msg *msg)
 {
-	u8 reg = msg->reg[0] & 0x7f;
+	u8 reg = msg->reg[1] & 0x7f;
 
-	if (msg->reg[0] & 0x80)
+	if (msg->reg[1] & 0x80)
 		ctx->xdevcap[reg] = msg->ret;
 	else
 		ctx->devcap[reg] = msg->ret;