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