V4L/DVB (4627): Vivi crashes with mplayer

The vivi.current_norm field is not initialized in vivi.c, so a
VIDIOC_G_STD ioctl without a prior call to VIDIOC_S_STD gives
unpredictable results. mplayer does exactly this.

Signed-off-by Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>