s5k3l6xx: Digital gain up to 3x

Higher values do make some difference, but they don't seem to be of any practical use. The datasheet recommends 3x (0x300) as a maximum, so I put it here to have more reasonable slider range in GUI.

There's still a thing with the minimum value, which should be 0x100 according to the datasheet, but in my experience setting it lower can somewhat help with taking low light photos. Slowing down the frame rate for higher integration time would probably be a better idea for low light, but we don't have that yet, so I didn't adjust the minimal value yet as the current limit may still be helpful.

btw, for this I'm just waiting for an Ack at least from @dorota.czaplejewicz because it changes the driver she's maintaining

Thanks for pinging.

added 23 commits

• 431f3b33...2f76e6e8 - 22 commits from branch Librem5:pureos/byzantium
• 7476e8a6 - s5k3l6xx: Digital gain up to 3x

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