Currently, ret is being checked for an error condition however it
is not being assigned in the previous statement on the call of
function mipi_dsi_dcs_exit_sleep_mode.  Add in the missing assignment
of ret.

Detected by CoverityScan, CID#1470174, 1470178 ("Unchecked return value")

Fixes: 26aec255 ("drm/panel: Add Ilitek ILI9881c panel driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180626160354.1363-1-colin.king@canonical.com