V4L/DVB (5816): Cx88-blackbird: fix vidioc_g_tuner never ending list of tuners

v4l-info and other programs would loop indefinitely while querying the
tuners for cx88-blackbird cards.

The cause was that vidioc_g_tuner didn't return an error value for
qctrl->id != 0, making the application think there is a never ending
list of tuners...

This patch adds the same index check as done in vidioc_g_tuner() in
cx88-video.

Signed-off-by: Jelle Foks <jelle@foks.8m.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>