Fix ADC_CFG in ADCInit() and correct comments.

[limingjie]

Fix #46.

1. Correct the ADC_CFG set statement in ADCInit().

ADC_CFG = (ADC_CFG & ~bADC_CLK) | speed;

2. Fix the comment according to CH554 datasheet.

ADC reference clock frequency selection bit 0: Slow clock. 384 Fosc cycles required for each ADC; 1: Fast clock. 96 Fosc cycles required for each ADC

[cibomahto]

Thanks!