[media] tda10071: do not get_frontend() when not ready

This is a bit hack, but returning error when driver is not tuned yet
causes DVBv5 zap stop polling DVBv5 statistics. Thus return 0 even
callback is called during invalid device state.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>