Fix auto-exposure

Currently on the PinePhone Pro the autoexposure code will increase one of the sensor controls to a point where the sensor will start integrating frames and drop the framerate to something useless for preview. This is probably the exposure time control but this has not been verified.

Maybe it's required to add gain/time limits as an option to libmegapixels to override the range reported by the V4L control if it switches behavior at some points in the value range. The integrated mode on the PinePhonePro sensor is pretty neat though to implement a night mode.