drivers/adc: stm32: avoid disabling and reenabling the ADC multiple times

The ADC is currently disabled and re-enabled multiple time during each
read. Enabling the ADC is not a free operation on some series and some
internal channels like VREFINT take even longer. This patch improve the
situation by removing the calls to adc_stm32_enable(), replacing it by a
single call just before triggering the conversion.

This also open the possibility to not reconfigure the ADC if it is
already configured, implemented in the following patches.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>