hi846: Fix the analog gain register address

The datasheet mentions 0x0076 on page 23, but the register table
lists analog gain under 0x0077 and it's the one that actually
makes the difference in the resulting image.

Closes #314
2 jobs for analog-gain in 65 minutes and 21 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed build:aarch64 #345481
librem5

00:33:14

passed build:deb #345482
librem5:arm64

01:05:21