[media] mt9v011: Fixed gain calculation

[media] mt9v011: Fixed gain calculation

The implementation of the gain calculation for this sensor is incorrect.
It is only working for the first 127 values.

The reason is, that the gain cannot be set directly by writing a value
into the gain registers of the sensor. The gain register work this way
(see datasheet page 24): bits 0 to 6 are called "initial gain". These
are linear. But bits 7 and 8 ("analog multiplicative factors") and bits
9 and 10 ("digital multiplicative factors") work completely different:
Each of these bits increase the gain by the factor 2. So if the bits
7-10 are 0011, 0110, 1100 or 0101 for example, the gain from bits 0-6 is
multiplied by 4. The order of the bits 7-10 is not important for the
resulting gain. (But there are some recommended values for low noise)

The current driver doesn't do this correctly: If the current gain is 000
0111 1111 (127) and the gain is increased by 1, you would expect the
image to become brighter. But the image is completly dark, because the
new gain is 000 1000 0000 (128). This means: Initial gain of 0,
multiplied by 2. The result is 0.

This patch adds a new function which does the gain calculation and also
fixes the same bug for red_balance and blue_balance. Additionally, the
driver follows the recommendation from the datasheet, which says, that
the gain should always be above 0x0020.

Tested-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Johannes Obermaier <johannes.obermaier@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>