V4L/DVB: gspca - ov534: Fix autogain control, enable it by default

* Use 'agc' instead of 'autogain' in the code so to align the naming
    as in AEC/AWB.
  * Tweak brightness and contrast default values.
  * Fix setting/resetting registers values for AGC.
  * Set actual gain back when disabling AGC.
  * Skip setting GAIN register when AGC is enabled.
  * Enable AGC by default.

Note that as Auto Gain Control is now enabled by default, if you are
using the driver for visual computing applications you might need to
disable it explicitly in your software.

Signed-off-by: Max Thrun <bear24rw@gmail.com>
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>