[media] cxd2820r: do not allow get_frontend() when demod is not initialized

This fixes bug introduced by multi-frontend to single-frontend change.

Finally HAS_LOCK is got back!
We are not allowed to access hardware in sleep mode...
Chip did not like when .get_frontend() reads some registers while
chip was sleeping and due to that HAS_LOCK bit was never gained.

TODO: We should add logic for dvb-core to drop out illegal calls like that.

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