Fix auto exposure sometimes making one cam dark and another bright

[apirrone]

Sometimes during teleoperation, one camera becomes very dark, and the other very bright.
Most of the time, we have to restart for it to go back to normal, but we had one occurrence where turning on (or off) the lights made it go back to normal on its own.

To me, it looks like an auto exposure bug. The two cameras are on sockets CAM_B and CAM_C, which are hardware synced, so the exposure/gain parameters are shared, and computed from one source.

What's weird is that they should both always have the same values, so it should be impossible for one image to be dark and the other to be light when looking at the same scene only a few centimeters apart.