RFC #60: address some feedback.

- do not enforce a divisor encoding (besides being 16 bits).
- fix signature direction (the peripheral drives the interface).
- do not use abbreviated names (`rdy` is renamed to `ready`, etc).
- rename `Enable` to `Control` and add ResR0W0 padding bits.
- clarify the behavior of `Control.enable` values (0 and 1).
- reorder registers (`Status` now follows `Control`).
- rename `data_bits` to `symbol_width` and `data` to `symbol`.
- `symbol_width` must be a positive integer.

Author: jfng