of: Reduce indentation in of_graph_get_next_endpoint

A 'return endpoint;' at the end of the (!prev) case allows to
reduce the indentation level of the (prev) case.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>