adc: test and sample failed on STM32

[erwango]

Describe the bug
Following issues were seen on adc driver during CI tests on STM32:

samples/drivers/adc/sample.drivers.adc, nucleo_l073rz
tests/drivers/adc/adc_api/drivers.adc, disco_l475_iot1
tests/drivers/adc/adc_api/drivers.adc, nucleo_h743zi

To Reproduce
Run twister on these using stm32 device

Expected behavior
Should be passed

Impact
Sample issue is a simple annoyance, but driver issue can prevent driver usage in some cases

Logs and console output

Running test suite adc_basic_test
===================================================================
START - test_adc_sample_one_channel
E: Only single channel supported

    Assertion failed at ../../src/test_adc.c:352: test_task_one_channel: (ret)
adc_read() failed with code -35

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows)
• Toolchain (e.g Zephyr SDK, ...)
• Commit SHA or Version used

Additional context
Add any other context about the problem here.