[PATCH] v4l: 721: check kthread correctly

- Check ->kthread correctly

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@m1k.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>