cmd: fix do_adc_single()

The source code contains an error:
- argv[2] contains <channel> arg, variable for env_set is in argv[3]
- number of args is 4

Revert 54d24d72
  cmd: simplify do_adc_single()

Fixes 9de612ae
  cmd: adc: Add support for storing ADC result in env variable
Reviewed-by: Simon Glass <sjg@chromium.org>