[media] cx18: fix VIDIOC_ENUMINPUT: wrong std value

The std field of v4l2_input is always V4L2_STD_ALL. For tuner inputs
this should be cx->tuner_std.

This fixes a v4l2-compliance failure.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>