drm/bridge/synopsys: dsi: readl_poll_timeout return value clean up

The readl_poll_timeout() return value is 0 in case of success
so it is better to detect errors without taking care of the
return value sign.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180204213624.18288-1-philippe.cornu@st.com