Add sample rate suitable to recovering bits at common baud rates.

TODO: Add general-purpose non-integer mode sample rate support back.
RFFC507x: Add function to read from readback register.
RFFC507x: Set register defaults to match programming guide v1.5, rev 1 parts.
Compile-time option to disable USB PLL/peripheral for PortaPack.
Slight type tweak to make C++ compiler happy.