V4L/DVB: [STV090x, STV6110x] Use tuner sleep within the demodulator control

Oliver Endriss <o.endriss@gmx.de> pointed out:
Imho not a good idea, as the frontend thread calls
- fe->ops.tuner_ops.init
- fe->ops.tuner_ops.sleep

If you remove fe->ops.i2c_gate_ctrl, init and sleep will fail,
because gate_ctrl was never called...

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Signed-off-by: Manu Abraham <manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>