temperature.cpp: Use OVERSAMPLENR in oversampling calculation.

/Marlin/blob/Marlin_v1/Marlin/temperature.cpp#L1193 is the end condition in the summation loop where the oversampling is applied.  I think this ought to be OVERSAMPLENR rather than '16'

Also, if I'm understanding the code correctly, Marlin triggers the TIMER0_COMPB_vect at count 128 of the default 16000000/64/256=976.5625Hz TIMER0, so the sampling period would be 8*16/(16000000/64/256)= 131ms instead of 128ms.