[media] r820t: add a commented code for GPIO

Add the code to set the GPIO for this tuner. This code is
currently unused, so it is kept there only for completeness.
With this patch there are just two things that got left from
the original driver:
- At standby, there's another mode, not used by rtl2832u.
  Not sure if it might be needed in the future, but I suspect
  it is not used at all;
- There is a "fast tune" mode. As nor DVB or V4L API supports
  it, it seems an overkill to implement it.

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