V4L/DVB (8904): cx88: add missing unlock_kernel

sparse found an unbalanced BKL usage.

Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/video/cx88/cx88-blackbird.c

···
 		err = drv->request_acquire(drv);
 		if(err != 0) {
 			dprintk(1,"%s: Unable to acquire hardware, %d\n", __func__, err);
+			unlock_kernel();
 			return err;
 		}
 	}