V4L/DVB (5117): Fix: VIDIOC_G_TUNER were returning an endless number of tuners

pvrusb2 have only one tuner inside. However, as it were not handling
index, a call to v4l-info were returning as if it were an infinite
number of tuners:
$ v4l-info|grep VIDIOC_G_TUNER |head -5
    VIDIOC_G_TUNER(0)
    VIDIOC_G_TUNER(1)
    VIDIOC_G_TUNER(2)
    VIDIOC_G_TUNER(3)
    VIDIOC_G_TUNER(4)

Acked-by: Mike Isely <isely@isely.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>