hi846: analogue_gain control has no effect

The selfie cam driver exports some V4L2 controls, like digital gain, exposure and analogue gain.

User Controls

                       exposure 0x00980911 (int)    : min=6 max=629 step=1 default=629 value=629

Image Source Controls

              vertical_blanking 0x009e0901 (int)    : min=151 max=65055 step=1 default=151 value=151
            horizontal_blanking 0x009e0902 (int)    : min=3160 max=3160 step=1 default=3160 value=3160 flags=read-only
                  analogue_gain 0x009e0903 (int)    : min=0 max=240 step=8 default=0 value=0

Image Processing Controls

                 link_frequency 0x009f0901 (intmenu): min=0 max=2 default=0 value=0 flags=read-only
                     pixel_rate 0x009f0902 (int64)  : min=0 max=0 step=0 default=0 value=32000000 flags=read-only
                   test_pattern 0x009f0903 (menu)   : min=0 max=9 default=0 value=0
                   digital_gain 0x009f0905 (int)    : min=0 max=8191 step=1 default=256 value=256

Adjusting them with v4l2-ctl works and changes the image produced (for example:)

v4l2-ctl -d /dev/v4l/by-path/platform-30a90000.csi1_bridge-video-index0 --set-ctrl digital_gain=8191 --set-ctrl exposure=2524

...except the analogue gain, which shows no difference in the produced image regardless of the value used:

v4l2-ctl -d /dev/v4l/by-path/platform-30a90000.csi1_bridge-video-index0 --set-ctrl analogue_gain=240