media: m2m-deinterlace: Fix misuse of strscpy

Probable cut&paste typo - use the correct field size.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>