[mythtv] NTSC v4l tuning above channel 45 broken after r6705

[Bryan Mayland]

After updating my repository this morning, tuning no longer works on my 
NTSC PVR-150 cards.  I tracked the problem down to the r6705 update from 
danielk. 

Channel::SetChannelByString is now selecting that the frequency for, 
say, NTSC cable channel 60 should be 60000 (due to the frequency = 
freqid.toInt(&ok) * 1000 + finetune).  isFrequency OKs any channel over 
45 and uses Tune() to change to frequency 60000 instead of using 
TuneTo(60, finetune).