drivers/ADC: STM32: This solves coverity reported in ADC driver.

This commit it to resolve following bugs:
* Operands don't affect result.
* Logical dead code in stm32_adc driver.
Above mentioned bugs were solved by adding parenthesis and
changed the method of comparing. Since comparison of ADC
channel_id with the channel may cause loss of value.
So instead of direct comparison, introduced a mechanism to
convert channel constant to a decimal using
__LL_ADC_CHANNEL_TO_DECIMAL_NB() and strips away
the INTERNAL_CH bit and then compare with channel_id.

fix:
* zephyrproject-rtos/zephyr#35130
* zephyrproject-rtos/zephyr#35136

Signed-off-by: Affrin Pinhero <affrin.pinhero@hcl.com>