[media] tuner-core: power up tuner when called with s_power(1)

Drivers must be able to rely on s_power to power up subdevices.

Note that at this moment no driver attempts to power up tuners. This probably
isn't surprising since s_power(1) was never implemented in tuner-core.c until
now.

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