drivers: adc: stm32: Refactor driver init section

Slightly refactor STM32_ADC_INIT macro.
adc_stm32_cfg_##index declaration is now part of ADC_STM32_INIT
and extracted from CONFIG_ADC_STM32_SHARED_IRQS #ifdef.

Aim is to minimize code duplication when adding new adc_stm32_cfg_X
or adc_stm32_data_Y entries

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>