uhd: Fixed "reducing chan number" bug

In some cases, there could be an error when reducing the number
of chans for UHD device in GRC (e.g. going from 2 chans to 1).
This adds another check which will alert the user.
filter: allow small rounding error in oversample rate setting for pfb_channelizer.
filter: fixes tag propagation through the pfb_channelizer.