video: of: display_timing: correct display-timings node finding

of_get_display_timing(s) use of_find_node_by_name
to get child node, this is incorrect, of_get_child_by_name
should be used instead. The patch fixes it.
Small typo is also corrected.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>