[media] usbvision: remove power_on_at_open and timed power off

This causes lots of problems and is *very* slow as well.

One of the main problems is that this prohibits the use of the control
framework since subdevs will be unloaded on power off which is not allowed
as long as they are used by a usb device.

Apparently the reason for doing this is to turn off a noisy tuner. My hardware
has no problem with that, and I wonder whether the hardware with that noisy
tuner wasn't just functioning improperly as I have never heard of noisy tuners.

Contact me if you have one of those devices and I can take a look whether the
tuner can't be powered off if necessary by letting the tuner subdevice go
into standby mode. Unloading the tuner module is just evil and is not the
right approach.

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