V4L/DVB (4995): Vivi: fix kthread_run() error check

The return value of kthread_run() should be checked by IS_ERR().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>