readpin: change sleep period from (10^9 / hw->freq) ns to (10^9 / hw->freq^2 * realfreq) ns
readpin: change sleep period from (10^9 / hw->freq) ns to (10^9 / hw->freq^2 * realfreq) ns

For hw-> freq = 1000 Hz, this improves hw->realfreq from 848 Hz to 910 Hz
and 858 Hz to 920 Hz with tunetime.

rene0/dcf77pi,rene0/dcf77pi