drivers: adc: stm32 adc driver must wait about 1ms after enabling

After enabling the ADC, the peripheral has a certain delay (about 1ms)
to set its ADC Ready flag in the ADC ISR register.
In between, the ADRDY is still 0 and the ADEN is 1 in the CR.
The ADC can be used for conversion, only when the ADRDY bit is set

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