V4L/DVB (4628): Fix VIDIOC_ENUMSTD ioctl in videodev.c

Do not return -EINVAL for index=0 in VIDIOC_ENUMSTD, because it is a
valid index

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>