drivers: sensor: stm32_vbat: handle disabled ADCs more gracefully

This commit modifies the STM32 battery voltage sensor driver
to handle erroneous usage more gracefully. More precisely,
it now fails 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>