media: tvp5150: Fix wrong return value of tvp5150_parse_dt()

If of_graph_get_endpoint_by_regs() return NULL, it will return 0 rather
than an errno, because we doesn't initialize the return value.

Fixes: 0556f1d5 ("media: tvp5150: add input source selection of_graph support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>