[media] v4l2-ioctl.c: fix enum_freq_bands handling

If the driver supports enum_freq_bands, but only for certain device
nodes, then it may return -ENOTTY. But in that case the code should
fall into the fall-back case where the current tuner/modulator range
is returned.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>