Remove Maybe from DataLink

[lmbollen]

Currently the DataLink type used by the scatter unit and gather unit is:

-- | Either contains a @Just (BitVector frameWidth)@ or @Nothing@.
type DataLink frameWidth = Maybe (BitVector frameWidth)

But since all communication is scheduled ahead of time we don't need any flow control mechanisms at this level.
As a workaround we can apply Just to all BitVector frameWidths and use fromMaybe 0 to strip them.

The scatter engines, gather engines, switch and their tests should be adjusted to reflect this change.