drm/panel: ili9881c: Fix missing assignment to error return ret

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