media: staging: ipu3-imgu: Fix BNR wb gain documentation

The documentation states that the BNR factor is a multiplier coded as
u3.13 and with a range of (0, 8). This is not correct, as the isp is
adding 1.0 to the gain applied, ie Pout = { Pin * (1 + Gx) }.

It means that a gain of 1.0 should be coded as 0.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>