Fix GCC fall-through warnings.

Fix GCC fall-through warnings.

When compiling ftdi.c in the context of blackmagic debug, more enabled
warnings and -Werror made me to add appended fixes.

Fixing the signedness of baudrate would probably be more logically by
making all baudrate and clk unsigned, but would need more code changes.
I decided to make clk signed as a fix with minimal
impact. The FTDI chips are far away from 2 GHz clock rate...