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.
2 jobs for pureos/byzantium in 65 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #353279
librem5
build:aarch64

00:37:24

passed #353280
librem5:arm64
build:deb

01:05:05