drivers: adc: stm32f1 adc driver enable peripheral

This commit avoid re-enabling the ADC,
because this starts conversion.
According to the RefMan (RM0008) of the stm32F10x mcu,
enabling the ADC will start the conversion
if the ADC is already enabled. "Conversion starts when
this bit holds a value of 1 and a 1 is written to it."
That's not what we want.

Signed-off-by: Francois Ramu <francois.ramu@st.com>