media: bttv: fix WARNING regression on tunerless devices

Commit 21615365 ("media: media/pci: set device_caps in struct video_device")
introduced a regression: V4L2_CAP_TUNER is always present in device_caps,
even when the device has no tuner.

This causes a warning:
WARNING: CPU: 0 PID: 249 at drivers/media/v4l2-core/v4l2-ioctl.c:1102 v4l_querycap+0xa0/0xb0 [videodev]

Fixes: 21615365 ("media: media/pci: set device_caps in struct video_device")
Signed-off-by: Ondrej Zary <linux@zary.sk>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>