Prior to this commit, the ADC calibration code was never executing because
ADVREGEN bit was set making the CR register always non-zero.

This commit changes the logic so that ADC calibration is always run when
the ADC is disabled and an ADC channel is initialised.  It also uses the LL
API functions to do the calibration, to make sure it is done correctly on
each MCU variant.
Signed-off-by: NDamien George <damien@micropython.org>