media: strscpy() returns a negative value on failure unlike strlcpy().

strscpy() returns a negative value on failure unlike strlcpy(),
so fix the WARN_ON accordingly.

Signed-off-by: Hans Petter Selasky <hps@selasky.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: added commit message]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>