V4L/DVB: gspca - t613: Change the gain mechanism

- variable / function rename
- set the gains in one exchange
- don't alter the register 80 which contains the AWB flag
  and other sensor specific values
- the global gain is now the average of the R, G and B gains.

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>