V4L/DVB (4743): Fix oops in VIDIOC_G_PARM

The call to v4l2_std_construct() in the VIDIOC_G_PARM handler treats
vfd->current_norm as if it were an index - but it's not.  The result is
an oops if the driver has no vidioc_g_parm() method defined.  Here's the
fix.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>