[media] VIDIOC_ENUM_FREQ_BANDS fix

When VIDIOC_ENUM_FREQ_BANDS is called for a driver that doesn't supply an
enum_freq_bands op, then it will fall back to reporting a single freq band
based on information from g_tuner or g_modulator.

Due to a bug this is an infinite list since the index field wasn't tested.

This patch fixes this and returns -EINVAL if index != 0.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>