hi846: Set digital gain range to 1.0x - 15.99x
Digital gain registers are comprised of 13 bits. 4 bits [12:9] control the integer portion, and 9 bits [8:0] control the decimal portion. Therefore 256 (0x100) corresponds to ~0.5x digital gain. This sets digital gain to 1.0x by default and limits the minimum value to 1.0x since there's little point in having global digital gain of less than 1.0x as its only effect is lowering picture quality by reducing its dynamic range and white point.