V4L/DVB (4928): Usbvision_v4l2 robustness on disconnect

This patch corrects 2 bugs (causes kernel oops) that occur when
unplugging the peripheral whereas nobody has opened it yet :
- do not call usbvision_stop_isoc if usbvision_init_isoc has not been called
- do not call wakeup_interruptible on waitqueues that did not have been
initialized with init_waitqueue_head

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>