drivers: adc: stm32: Fix sequence configuration
drivers: adc: stm32: Fix sequence configuration

The STM32 driver currently does not support reading a sequence of
multiple ADC channels. Only the first channel of the sequence was
read and the rest was silently ignored.

Fix: Return an error if reading multiple channels is requested.

Signed-off-by: Martin Jäger <54669547a225ff20cba8b75a4adca540eef25858@libre.solar>

Vudentz/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr