Fix usage of ADC bits for old FPGA/ADCs.

The configuration bits which control which 16-bit window to select from the 18-bit ADC outputs are being misused. The FPGA code for the new (sm2+) ADCs drops support for the old 16-bit ADCs, but the C code which controls both old and new ADCs allows setting the configuration to enable 16-bit support in the old ADCs, and swapped the usage of two of the bits. Not a problem for the new ADCs, but bad for the old ones.

A two-line fix plus a comment. Needed for SM1 at Subaru until the ADCs get swapped.