drivers: sensor: stm32_vref: handle disabled ADCs more gracefully

This commit modifies the STM32 internal voltage reference sensor
driver to handle erroneous usage more gracefully. More precisely:
  - driver no longer builds if no ADC node is enabled
  - fail builds with an explicit error message when the sensor
    is enabled but the corresponding ADC is not. This can only
    happen on STM32 series with more than one ADC (e.g., H7).

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>