cmd: simplify do_adc_single()

If argc is not < 3, it must be >= 3.

If argc >= 3, argv[2] cannot be NULL.

Fixes: 9de612ae4ded ("cmd: adc: Add support for storing ADC result in env variable")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
1 file changed